Inter-block gap
The blank stretch of tape between two data blocks, long enough for a start-stop transport to brake and get back up to speed.
Tape isn't addressed like disk; data is written in blocks separated by an inter-block gap (also inter-record gap). On a classic start-stop drive the gap — around 0.6 inch on 9-track — gave the capstan room to decelerate after a block and reach full speed before the next, so the head only read while the tape moved at rated speed.
Big gaps waste tape: many small blocks could burn more tape on gaps than on data, which is why software blocked many logical records into one physical block. Streaming drives that never stop between blocks shrank the gap dramatically. A longer erased stretch — the tape mark — signals end-of-file.