Block height gap refers to the difference in the number of blocks recorded on two different blockchains. This situation typically arises during events such as a blockchain fork, where a single blockchain splits into two separate paths due to differing protocol changes or consensus disagreements.

When a fork occurs, one version of the blockchain may continue to grow independently, leading to its own block height. Meanwhile, the original chain continues its own development. The gap between the block heights of these two chains signifies the divergence in their transaction records and state.

Block height gap can also be relevant in discussions about network security, as a significant gap may give clues about the relative strength or popularity of one chain over the other. Generally, a smaller gap may indicate that both chains have similar levels of activity or support, while larger gaps can point to one chain being more widely adopted or used.

Understanding this gap helps users navigate the complexities of blockchain evolution and network dynamics.