Whether a number or something else is even or odd.
So one can rephrase the question is 7 even or odd? as What is the parity of 7?
The parity of a permutation is the parity of the number of transpositions into which the permution can be decomposed. For example (ABC) to (BCA) is even because it can be done by swapping A and B then C and A (two transpositions).
Particles have a quantum mechanical property of parity. According to quantum mechanics, the conservation of this parity is equivalent to the laws of physics being invariant under mirror reflection. It has be found that parity is not conserved in the weak force and so the weak force is not invariant under mirror reflection.