Latency

Latency is the round-trip time between sending a trading message and receiving a response. In high-frequency trading it's the whole game — sub-millisecond matters — because whoever acts on new information first captures the opportunity before anyone else reacts. The usual stack of speed tricks: colocation (rent a rack inside the exchange), dedicated fiber (the infamous Spread Networks Chicago-NJ line), FPGA/ASIC hardware in the order path, kernel-bypass networking. For everyone else latency is measured in tens to hundreds of milliseconds and mostly doesn't matter.