A proposition is a proposed plan of action, a detailed suggestion. You might consider your friend's suggestion to set up a neighborhood snow-shoveling business after the blizzard a winning proposition.
The noun proposition means something presented for consideration. In some contexts, that something being offered is sexual. Therefore, don't use the term proposition loosely. If you say that you made a proposition to your teacher, people could misinterpret you. It's safer to stick with suggestion.