Tick rate
How many times a second a game server steps its simulation and samples input — 64 or 128 in the endless CS arguments.
A server doesn't update continuously; it advances in discrete ticks, each a fixed slice of time where it ingests inputs, steps physics and broadcasts state. The tick rate (in Hz) sets how fresh that loop is: 64-tick means 15.6 ms of granularity, 128-tick halves it — which is why competitive shooters argue about it endlessly.
It's independent of your render framerate — a 300 FPS client still only matters to the server 64 times a second. Higher rates sharpen hit detection and feel but cost CPU and bandwidth per player.