is a plant that grows on its own, rather than being deliberately planted by a human farmer or gardener.
A plant from a previous season's crop that regenerates in a subsequent crop, eg. potato tuber.
a planted bird that suddenly flies, on its own, without being flushed by the dog
plants which survive into the following growing season and which might then become a weed problem in a different crop planted for that season
Any plant that shows up in ones garden that wasn't purposely planted. Tomatoes are famous for being volunteers as an old tomato gets mixed into the garden in the fall and seeds sprout and pop up unsolicited in the spring.
Although this could be your best friend, helping out once in a while in the garden, this is a "not planted" specimen blown in from a neighbor's yard or a surprise, from a bird or other source.