Someday means at some point in the future, though exactly when isn't clear or decided yet. When you say you'll do something someday, you mean it will happen eventually, but you're not setting a specific date.

People use someday for things they hope or plan to do but can't do right now. A student might say, “Someday I want to visit Japan,” or “Someday I'll learn to play the guitar.” These are real wishes, but they require time, money, or skills the person doesn't have yet.

The word can express genuine intention: “Someday I'm going to read all the books in this series.” But it can also be a way of putting something off indefinitely. When someone keeps saying “someday” about the same goal year after year without taking any steps toward it, that someday might never come.

Someday captures the feeling of looking ahead to future possibilities. It's optimistic but vague, which makes it useful for talking about things you genuinely want but aren't ready to commit to yet.