Native Plants is a term to describe plants endemic (indigenous) or naturalized to a given area in geologic time.
Some would question that part of this definition which holds that species which have become naturalized in a given area can become "native". There's shades of gray here. For example, something like water lettuce, which has been in S. Florida since the 1500s has "become" native in some people's minds, although it got there courtesy of Spanish explorers-just a long time ago.
And then there's the simple, yet oft-ignored fact that nature isn't static. Even without human interference, species have moved to and colonized new areas for millions of years. I'm reminded of the formation of new volcanic islands off the coast of SE Alaska. Featuring fertile soil and a blank slate so far as vegetation is concerned, such locations have been known to get colonized by coniferous tree species very readily. Should we decry this because the "native" condition was one of bare rock and ashy soil? I think most would agree, no we should not. There are tougher examples out there than this one though.