The maximum amount of data a block can contain. Originally set to 1mb by Satoshi as a spam prevention measure, the block size limit has been the source of a lot of controversy as it keeps the network operating efficiently however prevents the base layer from scaling beyond ~2k transactions per block.
The debate to increase the block limit size resulted in a fork of both the bitcoin chain as well as the community which resulted in the creation of Bitcoin Cash which has a 32MB block size. In the 2017 SegWit update, some block data was moved outside of the block essentially increasing the number of transactions per block 4 fold without increasing the 1mb limit. For a detailed explination of this contriversial topic, be sure to read 
The Blocksize War: The battle over who controls Bitcoin’s protocol rules
Word missing from the glossary or definition needs improvement? Let us know!