Lamport clock
A logical clock without synchronised time — counts events to produce a "happened-before" ordering in distributed systems.
Leslie Lamport's 1978 paper "Time, Clocks, and the Ordering of Events". Rules: (1) every event bumps the local counter, (2) sent messages carry the counter value, (3) the receiver sets its counter to max(own, received) + 1. Guarantees that if event A "happens before" event B, A has a lower Lamport timestamp. Limitation: doesn't give total ordering (can't distinguish concurrent events). The fix there is vector clocks. Foundational for every distributed consensus algorithm.