Cut-out
From wipipedia.org
A cut-out is a large hole deliberately cut in a piece of clothing to reveal the wearer's bare flesh underneath. The commonest place for a cut-out is over the breasts to emphasise cleavage.
A similar idea is to have a long slit in a dress or skirt from the hem upwards, so that even though the skirt is not short, the thigh may be glimpsed as the wearer moves.
