Definition
Cache-aside (also called lazy loading) is the most common caching pattern: the cache is passive, and the application owns the logic. On a read, the application checks the cache; on a miss it queries the database, writes the result into the cache, and returns it. Subsequent reads hit the cache until the entry expires or is invalidated.
This differs from read-through caching, where the cache layer itself fetches from the database, and from write-through/write-back, which involve the cache on the write path. Cache-aside's virtues are simplicity and graceful degradation — if the cache dies, reads still work (slower) against the database. Its costs are the classic invalidation problem (the application must expire or update entries when data changes) and the first-request miss penalty.
How it comes up in interviews
When you say "we'll add Redis", the follow-up is "what happens on a miss, and how does an update invalidate?" Naming the pattern, stating TTL-plus-explicit-invalidation, and mentioning what happens if the cache is cold (see cache stampede) covers the expected ground.