If you have a notion that you can swim across the ocean, you are probably wrong. A notion is an idea, often vague and sometimes fanciful.
A notion is lighter than a theory and embraces a whimsy that a simple idea never could. You might have the notion that you can slow global warming by taking public transportation and reusing plastic bags. If you share a far-fetched idea with others, someone might respond with a "Where'd you get that notion?!" Notions can also refer to the buttons, thread, zippers, and pins necessary for sewing things. So if you have a notion to create your own Halloween costume, you better pick up some notions from the craft store.