I've been wondering this for a while and I couldn't get a straight answer, so naturally I come to 4chan for help.

When I was in middle school, I was taught that an organism is a plant if it's multicellular, eukaryotic, and autotrophic. In high school I learned plants had to have cell walls made of cellulose, too. But I've since learned that things such as the "corpse lily," rafflesia arnoldii, are counted as plants even though they don't perform any photosynthesis at all, and may lack things like stems or leaves. Furthermore, things like seaweed, despite all appearances and the fact that they photosynthesize, are not plants.

So what exactly IS a plant?