A yump is a jump off of the large switches in the switch palaces. Mario is able to jump off the switch only on the frame it is pressed, making this a frame-perfect trick.
P-switch jumps are sometimes incorrectly called yumps. This abuse of terminology should be avoided. While both are frame-perfect jumps off of hemispherically shaped objects which depress when Mario lands on them, that is the extent of the similarities between these two actions.
The exact origin of the term is unknown, but it is conjectured to be of Scandinavian origin. In several languages including Swedish, the letter j is a voiced palatal approximant, which in English is usually expressed by the letter y (as in "yoyo"). Legend has it that some Swedish runner in the early days of SMW would attempt this trick during runs and exclaim "jump!" when it was successful, which would be heard by English-speakers as "yump". While this may be correct, attempts to locate this mythic progenitor have not been successful; the true story may be lost to history.
There is a highly innovative vanilla SMW romhack named after this trick: YUMP