a hole, opening, or break in something or between two things;
e.g. The mouse went through a gap in the wall.
(noun)
a space or separation that is empty and unfilled, suggesting that it is deficient or incomplete;
e.g. There is a gap in the knowledge we have on this subject.
(noun)
a break in the balance or continuity of something, or a wide difference in attitude, nature, situation, or viewpoint, especially one that is undesirable;
e.g. There is a gap between the people supplying resources and the people who need the resources.