Chiefly means mainly or mostly, but not entirely. When something is chiefly true, it's true in most cases but there might be a few exceptions. If a book is chiefly about dolphins, dolphins are the main subject, though it might mention whales or sharks too.

You'll often see this word in reference books and encyclopedias. A plant that grows chiefly in tropical regions is found mostly in hot, humid places, but perhaps also in a few subtropical areas. An author who writes chiefly for children focuses mainly on young readers, though they might occasionally write for adults.

The word helps writers be precise and honest. Instead of saying “only” or “always” (which might not be completely true), they can say chiefly to show that something is mostly true while leaving room for exceptions. If someone says your state is chiefly agricultural, they mean farming is the main industry, even though cities and factories exist there too.

Think of chiefly as a more formal version of “mostly” or “mainly.” It means the same thing but sounds more scholarly, which is why you see it often in textbooks, nature guides, and informational writing.