1. An opening in a solid structure or surface: Henry's dog wriggled through a gap in the fence at the backyard when it came back from snooping around.

There was a large gap in the wall where the artillery shell exploded.

2. A pass or access through large steep hills: When Tim and his friends were exploring a geographic region, they were able to walk through a gap in the mountains so they could see what was on the other side.
3. A space between objects or points; an aperture: Shirley had an abnormal gap between her two front teeth.
4. A conspicuous difference or imbalance; a disparity: There was a big gap between Sue's income and her spending during the last month.

In a semiconductor, the energy difference between the highest valence band and the lowest conduction band.

The amount of energy (in electron volts) required to free an outer shell electron from its orbit about the nucleus to a free state, and thus promote it from the valence to the conduction level.

A difference between what is claimed to be true and what is actually true. (1)

One of a series of vacancies in the distribution of the orbits of the minor planets, marked by the absence of minor planets whose orbits have periods that are simple fractions of the orbital period of Jupiter.

The reason for this activity is that in such positions any minor planet would be repeatedly perturbed by Jupiter's gravitational field until it was forced out of the "forbidden" orbit.

The gaps are an example of gravitational resonance.