HTLC is an acronym that stands for Hashed-TimeLock-Contract
A Hashed TimeLock Contract is a class of payment instruments (commonly built as a compilation of smart-contract in the cryptocurrency industry) that allows for the opening of private payment channels between contract participants. Those constructed payment channels are able to provide more advanced and flexible financial services such as conditional payments.
HTLC’s are commonly used as a core building block of 2nd layer scaling solutions for blockchains.