Thistle is the common name of a group of flowering plants characterised by leaves with sharp prickles on the margins, mostly in the family Asteraceae. Prickles occur all over the plant – on the stem and flat parts of leaves. They are an adaptation that protects the plant from being eaten by herbivores. Typically, an involucre with a clasping shape of a cup or urn subtends each of a thistle's flowerheads.
The term thistle is sometimes taken to mean exactly those plants in the tribe Cardueae (synonym: Cynareae), especially the genera Carduus, Cirsium, and Onopordum. However, plants outside this tribe are sometimes called thistles, and if this is done thistles would form a polyphyletic group.
Thistle is the floral emblem of Scotland and Lorraine, as well as the emblem of the Encyclopædia Britannica.