Imaginable means able to be thought of or pictured in your mind. If something is imaginable, you can conceive of it, even if it doesn't exist yet or seems unlikely to happen.

When a scientist considers every imaginable solution to a problem, she's thinking through every possibility she can dream up. When someone says “the worst day imaginable,” they mean the absolute worst day they can picture. The word often appears in phrases like “every imaginable color” or “the best meal imaginable,” suggesting the full range of what's possible to think about.

The opposite is unimaginable: so extreme, strange, or unprecedented that your mind struggles to picture it. Before the Wright brothers flew at Kitty Hawk, powered flight seemed unimaginable to most people. Once they succeeded, it became not only imaginable but achievable.

Notice how imaginable connects to boundaries of thought. When you read science fiction about colonies on Mars, the author is exploring what's imaginable about future technology and society. When you plan your dream treehouse, you're working within what's imaginable given your materials, skills, and the tree in your backyard. Imagination lets you explore possibilities, and imaginable describes everything that exploration might reach.