When using the participation method 'Collect input and feedback' in a project the input you are collecting is called 'Idea' by default.
There are several other options you can choose on your platform to rename the inputs.
In addition to ideas, you can also choose Option, Project, Question, Comment and Contribution. So, you don't have to invite people to "Post an idea," you can also ask them to "Submit a question" or "Add a comment."
If you're on a standard or premium plan, you'll see the option to specify "What should an input be called?" when you're setting up your project.