Microservices are service/s that are significantly smaller than the service you're comparing them to. That's an obvious and perhaps unhelpful definition, let's contrast it with ...

"Microservices" is[0] a tech industry fashion trend. It can be a real solution to a problem, too, but it's like how throwing out all your shit is the solution to depression now because a nice lady in a book said that you probably don't want all that stuff anyway. The solution is applied/suggested because it's trendy, not because it is necessarily the best fitting solution.

Like many buzzwords, 'microservice' is difficult to define. "You know it when you see it" is a euphemism for something devoid of meaning[1]: a null concept. This is a feature and not a bug. And where the term does have meaning, it's always relative -- so everyone can have microservices, and argue about who does.

[0] I do mean 'is', as opposed to 'are'

[1] it can also be the case that the speaker just doesn't know what they're talking about, and perhaps that they know it has meaning to others, social constructs, &c. To avoid spending the next three years working on this comment, I'm assuming that's not the case above.

[x] Conspiracy-theory rabbit-hole: buzzwords are like conspiracy theories in that they rely on vagueness or being undefinable. I guess my point is that concrete ideas aren't really suited to being buzzwords. Only the vague/relative/broad ones get selected (as in natural selection, not by the shady cabal of buzzword pickers), just like how only the best impossible contradictory stories tend to become popular conspiracy theories. ... actually, mostly I just thought "conspiracy-theory rabbit-hole" was a funny heading for a tangent about conspiracy theories.