I think in this case about DNS caching, "long tail" is better than "infrequent". In the wikipedia graph, some of the domain lookups in yellow may be "frequent" (absolute sense) but simultaneously but much less popular (long tail) such that they don't stay in DNS lookup caches.
DNS is essentially a cache. I've never once in my life heard of infrequently accessed cache items as "long-tail". This is definitely a dumb phrase that should be avoided.
You also used the word "domains" when you asked about "infrequently used domains" and that's the context I was responding to. I didn't say that cache items are labeled "long tail".
You could call them "infrequent" but "long-tail" is also a common description to convey a Power Law distribution: https://en.wikipedia.org/wiki/Long_tail
I think in this case about DNS caching, "long tail" is better than "infrequent". In the wikipedia graph, some of the domain lookups in yellow may be "frequent" (absolute sense) but simultaneously but much less popular (long tail) such that they don't stay in DNS lookup caches.