A peak limiter uses extremely fast attack times (microseconds) and high ratios (∞:1 or 20:1+) to catch and reduce transient peaks before they exceed the ceiling. This is the essential mastering tool: set the ceiling at −1 dBTP, raise the input gain until you achieve the target loudness, and the limiter prevents any peak from exceeding the ceiling.
Lookahead limiters analyze the signal slightly ahead of time, allowing them to catch peaks that would slip through a real-time limiter. This adds a small amount of latency but provides cleaner, more transparent peak control. Brickwall limiters are peak limiters designed specifically for mastering.