It took a while, but I finally got curious/sick enough of seeing ‘ponte’ everywhere in shops to look up what it actually is. I mean, I knew it was a stretchy fabric, but why call it ‘ponte’ specifically?
Turns out ponte is actually short for ‘ponte di Roma’, which means ‘Roman bridge’:
A fabric made in a double knit construction, usually produced in one color rather than color patterns. This plain fabric has an elastic quality with a slight horizontal line. The fabric looks the same on both sides.
Weft knitted, interlock based, double jersey structure. Means ‘roman bridge’ which is suggested by the arrangement of loops. The fabric looks the same on both sides.
I guess fashion terminology always tries to find a shorthand for things, but Ponte di Roma sounds much nicer than just ponte. When ‘ponte pants’ started turning up a couple of years ago I thought they sounded like a feminine hygiene product.