LRU Cache
In plain terms
A cache that evicts the least-recently-used item when full.
Going deeper
An LRU cache keeps a bounded set of items, discarding whichever hasn't been accessed longest to make room. It approximates keeping 'hot' data available.
Under the hood
Typically a hash map plus a doubly-linked list gives O(1) get and put; a classic interview and systems-design problem.
Examples
CPU/page caches, web caches, browser caches, memoization with limits.
Think of it like...
Like a small fridge: when full, you toss whatever you haven't touched in the longest.