>>10864909It is not a uniform meaning.
Usually it designates a subarea that has something to do with number theory, Galois theory or more generally the study of finite field extensions.
In algebraic geometry, usually you will dub a situation "geometric" if the objects of study are defined over an algebraically closed field, and call "arithmetic" phenomena those phenomena that arise specifically in the non algebraically closed case.
Arithmetic geometry is a loosely defined subfield of algebraic geometry that deals with "arithmetic varieties/schemes", that is varieties defined over rings of number theoretic interest (eg. global fields, or local fields, or their rings of integers).
Arithmetic combinatorics studies number theoretic questions that look combinatorial (so questions about the distribution of primes or the additive properties of the integers fit into this category). Basically it has a lot of overlap with analytic number theory
