To do something casually means to do it in a relaxed, informal way, without much planning, effort, or seriousness. When you mention something casually in conversation, you bring it up in a light, offhand manner, like you're not making a big deal of it. You might casually ask a friend if they want to hang out this weekend, without pressure or elaborate planning.

The word suggests ease and lack of intensity. A casual reader picks up books now and then for fun, while a serious student of literature reads deeply and analytically. Someone dressed casually wears comfortable, everyday clothes like jeans and a T-shirt rather than formal attire. A casual friendship is pleasant but not particularly close or committed.

You can also act casually to hide your true feelings. If you're nervous about trying out for the school play but don't want others to know, you might casually mention you're auditioning, as if it doesn't matter much to you either way. This kind of casualness is a social strategy: showing you care a lot can feel risky, so people sometimes pretend to be more relaxed than they really are.