that’s an idea—that thought, proposal, etc. is worth considering:
- Well, that’s an idea all right. Not a good one, but it’s an idea.
that’s the idea—
1. is used to indicate approval of what has just been said:
- “Shall I look and see?” “That’s the idea.”
2. that’s what smb. has in mind:
- Personal liberty isn’t everything. The collective utopian omelet cannot be made without breaking eggs or intellectuals … I think that’s the idea, and it’s still quite a bad one.