Bloom Filter
In plain terms
A compact structure that tells you if an item is *probably* present or *definitely* absent.
Going deeper
A Bloom filter uses several hash functions and a bit array to test membership using tiny memory. It can give false positives but never false negatives, so it's great for quick 'have we seen this?' checks before a costly lookup.
Under the hood
Tunable false-positive rate via array size and number of hash functions; can't delete elements (counting Bloom filters can). Saves huge memory at scale.
Examples
Bigtable/Cassandra to skip disk reads, Chrome's malicious-URL check, dedup pre-checks.
Think of it like...
Like a bouncer with a fuzzy memory: 'definitely never seen you' is reliable, but 'you look familiar' might be wrong, so you double-check the guest list.