BIP 141 (https://github.com/bitcoin/bitcoin/pull/8149) introduced a new metric for counting block size. This was initially called 'block cost', but was renamed to 'block weight' in https://github.com/bitcoin/bitcoin/commit/2c06bae39edfaa9c0855d83377ad8fda09e4fa08 (and is referred to as 'block weight' in https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki)
BIP 141 also changed the way that sigops are counted in transactions and blocks in exactly the same way - the limit was increased 4x, sigops in the main tx structure are counted 4x and sigops in the witness are counted 1x (essentially giving sigops in the witness a discount). This new metric was initially called SIGOPS_COST in the code. This PR changes the name to SIGOPS_WEIGHT to bring it in line with transaction/block weight.