Tap Pants, French Knickers or Scanties are high waisted panties that have a full, shorts style leg which is worn loose at the leg opening. This style of underpants was popular from the 1920s to the 1950s, with a retro resurgence in the 1980s.
Tap Pants is the term used in the US and derives from the trend in the 1930s-40s of showgirls wearing shorts of a similar style for dance rehearsals.
French Knickers is the term used in the UK. Scanties was the term used in Australia during the 1950s. Nobody seems to say scanties today except for older Australian women who had their heyday in the 1950s, who invariably still use the term for this garment.
Generally speaking, 1920s tap pants tend to be made of silk and are longer and fuller in the leg. 1930s tap pants tend to be bias cut and close on one hip with little buttons. They are also longish in the leg. 1940s and 1950s tap pants are often made of rayon, especially those made during the war years when silk was not available (it had all been requisitioned for the manufacture of parachutes). Legs tend to be shorter than 1930s pants. 1940s-50s tap pants may be bias cut, but in the 1950s were more likely to be a tad more full and gathered at the waist on elastic. There are of course many exceptions so these are guidelines rather than rules.
See my blog post about vintage knickers: