Legally speaking, what is an open source contribution?

Many companies have to be concerned about their employees contributing to open source projects for fear of creating or contributing intellectual property. Similarly, OSS projects need to keep their IP clean (copyrights etc) to encourage wide reuse. But what is a contribution? Is a +1 on a mailing list a contribution? Or a 200 line patch? Do you know one when you see one? Or is there some other metric you can use?