Hole
Beginner
An empty spot where an electron is missing. Nearby electrons hop into it, so the hole moves around like a bubble and acts like a positive charge.
Novice
The absence of an electron in the valence band, treated as a particle with positive charge. Holes are the majority carriers in p-type silicon and carry current just as electrons do.
Expert
Modeled with its own effective mass and mobility (Si: about 470 cm²/V·s in lightly doped material, vs 1400 for electrons). PMOS transistors conduct with holes, which is one reason they are drawn wider than NMOS for the same current.
Explained in Bands, doping and the p-n junction (Transistors).
See also: Valence band, p-type.