Answer
When a hot key expires and thousands of requests miss at once and all hit the database, it is a cache stampede. Letting one request rebuild the value while the others wait for it is called request coalescing.
Also accepted: thundering herd, dogpile for cache stampede; single flight, singleflight, coalescing for request coalescing.
Why
Other fixes: refresh hot keys before they expire, and add random jitter to TTLs so many keys do not expire in the same second.