Sure, but that’s wrong and used for convenience. Veganism is an ethical movement that sees this exploitation as fundamentally wrong. That was the original definition, but the Vegan Society who created the term changed it (in the 1940s?) to include avoiding exploitation generally “as far as is possible and practicable”.

What you’re referring to is a plant-based diet(er). Calling a vegan “someone who eats meat no more often than a month” in the sense of how actual vegans use the term would be like defining someone who’s anti-dog-abuse as someone who kicks their dog at most once a month.