The 1960s mid-1960s
The Mini Skirt
The single garment most synonymous with the entire decade - a hemline that climbed from just above the knee to genuinely thigh-length within a couple of years, and never fully retreated afterward.
Ask most people to name one thing about 1960s fashion and this is what they'll say, which makes it a slightly strange entry to write since there's very little left to reveal about the garment itself - it's a skirt, cut well above the knee, sometimes dramatically so by 1966-67. What's more interesting than the length is the argument over where it actually came from, because Mary Quant and André Courrèges both have real claims to it, and the disagreement itself tells you something about how this garment actually spread.
Quant's account was always that she was responding to what her young Chelsea customers were already doing to their own hemlines rather than inventing the length from scratch - a bottom-up, street-first story. Courrèges's 1964 collection presented a similarly short hemline as part of a fully worked-out couture vision, a top-down, designer-first story. Both are probably partly true, which is really the point: this is a garment that seems to have arrived from multiple directions at once, London boutique customers and Paris couture ateliers converging on the same length within the same year or two, rather than trickling down from one clear point of origin the way fashion "used to" work.
What I think earns the mini its place as the decade's defining garment isn't the length alone, though - it's how completely it reoriented the mod silhouette around a younger body and a faster, more disposable relationship to clothing generally, which is exactly the youthquake shift this whole era is remembered for. A hemline that short simply doesn't work with the structured, corseted foundation garments the previous decade relied on - wearing one required, almost by necessity, abandoning a whole undergarment logic along with the extra fabric. Very few single garments force that much downstream change in how a woman gets dressed underneath the visible layer.