Published in 1996, RIPEMD160, short for RIPE Message Digest, is one of the five variants of the RIPEMD hashing algorithims that outputs a 20-byte / 40-hex hash. It's included as a stand-alone op_code, but OP_RIPEMD160 is rarely, if ever, included in any common script. However, RIPEMD160 itself *is* used substantially through the more popular OP_HASH160 & OP_HASH256 operations.