Merriam-Webster's Collegiate® Dictionary, 11th Edition
GAP
I. noun
Etymology: Middle English, from Old Norse, chasm, hole; akin to Old Norse gapa to gape
Date: 14th century
1. a break in a barrier (as a wall, hedge, or line of military defense), an assailable position,
2. a mountain pass, ravine , spark gap ,
4. a separation in space, an incomplete or deficient area , a break in continuity ; hiatus , a break in the vascular cylinder of a plant where a vascular trace departs from the central cylinder, lack of balance ; disparity , a wide difference in character or attitude , a problem caused by some disparity , gappy adjective
II. verb (gapped; gapping)
Date: 1879
transitive verb to make an opening in, to adjust the space between the electrodes of (a spark plug), intransitive verb to fall or stand open