ANDROGYNY, as you may or may not know, is the fusion of both male and female characteristics in a single person.

That's the dictionary version, anyway. But if that's too complicated a definition, think of David Bowie singing Starman on Top Of The Pops on July 6, 1972, the moment that set a nation of impressionable young men on a collision course with their sisters' make-up bags.

Or, if you want a more literary take on it, dust down that old copy of Virginia Woolf's gender-bending 1928 novel Orlando. Its titular hero starts out a man in Elizabethan England, wakes up as a woman in Constantinople some years later then careers through the centuries until he/she hits the 1920s, a period in which, as Cole Porter would later note, anything goes.

Since then, androgynous figures both real and fictional have coursed through literature, music, film and art, and from there they have travelled both into popular culture and the sub-cultures beyond. Picture Grace Jones on the cover of her 1981 album Nightclubbing, for instance, or a suited KD Lang in a barber's chair being "shaved" by Cindy Crawford for that infamous Vanity Fair shoot. Picture Marlene Dietrich in top hat and tails in Morocco, or Tilda Swinton in, well, pretty much anything directed by Derek Jarman. Or, most bewitching of all, think of Patti Smith photographed by her friend and lover Robert Mapplethorpe for the cover of her 1975 album, Horses.

In Just Kids, her elegaic memoir of her time with the bisexual Mapplethorpe, Smith writes about the day he took a picture which remains one of rock's most iconic images. To prepare for it, she went to a Salvation Army shop in the Bowery and bought some men's white shirts and for the shoot itself she chose one monogrammed "RV" because it made her think of Roger Vadim, director of camp space odyssey Barbarella. To that outfit she added black trousers with the legs rolled up, black ballet shoes, white socks and a black jacket. Androgyny to the max, in other words.

But what really makes the shot, besides Smith's chopped hair and Modiglianian beauty, is what she did when Mapplethorpe said he wanted to see more of that monogrammed shirt. "I flung my jacket over my shoulder," she writes, "Frank Sinatra style. I was full of references."

These "references" were both male and female, of course, and well Smith knew it. It's why that picture and that look have proved so influential to designers ever since.

So when fashion and androgyny embark on one of their periodic love-ins - or "moments", as they say on Planet Couture - you can be fairly certain Mapplethorpe's snap of Smith channelling Ol' Blue Eyes is the first thing pinned up on the mood board. I imagine David Bowie-as-Ziggy Stardust soon joins her there.

We're having one of these androgyny moments now, as it happens. And, like everything else in the cyclical world of fashion, it's exactly the same as last time - the early 1990s - and also completely different.

Possibly it started with the rise of women wearing brogues. Or maybe it was with the advent of so-called "boyfriend jeans", the name applied to baggy, loose-fitting denims which lend their wearer the air of someone who didn't sleep in her own bed the night before and who has had to borrow some clothes as a result. (Ideally, boyfriend jeans are modelled in a cool Sunday brunch spot, at a cosy corner table which is shared with the jeans' owner, or in a cafe surrounded by every young woman's must-have lifestyle accessory: a family of supportive girlfriends. But if you don't have a boyfriend, don't worry. Gap sell the jeans without the man attached.)

The trend has continued, though, with over-sized men's shirts for women and the rise in popularity of Dr Martens. There are now over 300 women's styles available on the company website. And it may reach its zenith (or nadir, depending on how you look at it) with the fact that the words "boilersuit" and "fashion" aren't as mutually exclusive as they probably should be. If you don't believe me, Google those same two words. You'll find Top Shop and Kate Moss mentioned in two of the top three hits, which is as good a signal of style as anyone could possible want.

Earlier this month, meanwhile, British model-turned-actress Agyness Deyn launched Title A, a clothing line for women inspired by menswear. Deyn, who has previously collaborated with Dr Martens and is known for wearing mannish clothes, has set up the company with her sister and a friend. "We started craving things that we wanted to wear so much, which were menswear clothes without the feminine touch that people give them sometimes," she told New York Magazine. Among the style influences she cites are Mick Jagger, Kurt Cobain and, yes, Patti Smith.

Upmarket retailer Harvey Nichols is getting in on the act too. But as well as aiming to capitalise on these trends it thinks it has spied a growing appetite for unisex clothes and is promoting a "gender neutral" His Or Hers "edit" on its website.

Included in it are clothes by Givenchy, Versace and Alexander McQueen alongside pieces by high street retailers such as Whistles and Ralph Lauren. What's interesting as you click through the collection, though, is seeing how the feminine clothes look on male models and vice versa. You expect a woman to be modelling the Moschino leopard-print shirt or the shocking pink Versace T-shirt but instead it's a man. Likewise, you don't expect to see a woman in the faded red sweatshirt of the blokey green parka by wellie maker Hunter.

Backing up Harvey Nichols's hunch that gender-neutral clothes are "a thing" is an accompanying survey which suggests that 48% of Britons regularly wear an item of their partner's clothing. That rises to 77% among 18 to 25-year-olds. It isn't all one-way traffic either, as you might imagine it would be. We aren't quite at the stage where we'll see men wearing "girlfriend jeans" to the pub, but 34% of men do admit to having worn some item of clothing that isn't actually theirs and the advent of "girlfriend knickers" might not be too far off: more men than women admit to having worn their partner's underwear. Apparently.

So is it a case of lies, damned lies and surveys? Maybe. It's certainly true that even unisex clothing is far from new. They had it in the 1960s and if you dig out the sketches for Jean-Paul Gaultier's spring-summer 1991 collection - c'mon, I know you've got them somewhere - you won't see any marked difference between the men's and women's clothes. That's a pretty good definition of His Or Hers in my book.

But there are ways in which the 2014 androgyny trend is different because as well as letting women dress a lot like men, men dress a little like women and both sexes borrow clothes from each other, it's asking some deeper questions about the ambiguities surrounding gender. And it's doing this not so much through the clothes themselves, as through the way those clothes are marketed and, increasingly, who's wearing them on the catwalks.

Most provocatively, we've recently seen a man modelling women's clothes. He is (or was: more of that later) Bosnian-Australian model Andrej Pejic and he has featured in campaigns and fashion shows for Jean-Paul Gaultier, Marc Jacobs and DKNY among others. He has also modelled bridal wear for Spanish designer Rosa Clara, push-up bras for Dutch lingerie line Hema and, in 2011, was ranked 98th in FHM's 100 Sexiest Women.

In July of this year, meanwhile, he revealed that he had undergone a sex change and is now to be known as Andreja Pejic. In having the operation Pejic has resolved a troubling personal question, but what's notable is that it will make no material difference to her modelling career.

But while Pejic has tended to hog the limelight and the headlines, don't think there isn't an army of other gender-bending models out there. Supermodels such as Stella Tennant and Agyness Deyn herself have long traded on their adrogynous looks, and so have the designers for whom they act as muse. But among the less well-known names on the distaff side are women like Saskia de Brauw, Casey Legler and Lea T.

Born Leandro Cerezo in the Brazilian city of Belo Horizonte in 1981, and the son of Brazilian footballer and World Cup star Toninho Cerezo, Lea T also underwent a sex change in March 2012 but was a long-haired male when she began modelling for Givenchy in 2010 in a series of ambigous shoots.

Legler, meanwhile, competed for France in the pool at the 1996 Olympics in Atlanta then moved to New York where she works as an artist and as a model. She's on the books of the famous Ford agency - but as an exclusively "male" model. And Dutch model de Brauw was chosen by designer Hedi Slimane to front last year's Yves Saint Laurent spring-summer menswear campaign. She also joined Tilda Swinton and Andrej Pejic in the video for The Stars (Are Out Tonight), from the latest album by guess who? Yup, the original androgynous pin-up himself, David Bowie.

So if we learn anything from all this, it's that what goes around really does come around - and sometimes it's wearing its girlfriend's clothes when it does.