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Hashed TimeLock Contract (HTLC)

: A class of 

smart contracts that require that the receiver 

of a payment either acknowledge receiving 

the payment prior to a deadline by generating 

cryptographic proof of payment or forfeit the ability 

to claim the payment, returning it to the payer.

IOU

: A cryptographically-signed piece 

of data acknowledging a debt.

Implementation

: A specific realization of a protocol 

or other software abstraction in the form of one 

particular incarnation in particular software code. 

Loosely speaking, a blueprint is to a house as a 

protocol specification is to an implementation.