This fascinating "History of Picture Stories" has a page showing the evolution of speech balloons, the ubiquitous graphic convention used to convey that comic characters are saying something. The image seen here is detail of Bernhard Strigel's "Saint Anne and Angel" (1506/1607). From the Evolution of Speechballoons page:
During the 18th century, British caricaturists changed the shape of speechballoons from gothic speech-bands or flags into fluffy balloons, our modern speechballoons.
I'm using the word speechballoon as the general, inclusive term. (The gothic form of speechballoons are speechbands, flags, scrolls or sheets of paper, the modern form of speechballoons are balloons, but also little rectangles, often rounded at the edges, or simply little blocks of text above the heads of the speaker etc, etc).
The 18th century term for speechballoons was 'labels'.