|Definition - That which is grammatically correct, but which might not be meaningful.|
(1) Bob sent me a letter. (Well-formed and meaningful)
(2) Bob sent. (Not well-formed and not meaningful)
(3) Bob sent me stupidity.(Well-formed, but perhaps meaningless)
Oxford English Dictionary - The term's first citation in this sense is from 1961:
"One important common problem is that of obtaining an algorithm for distinguishing sentences from nonsentences or … well-formed strings from not well-formed strings."
(A. G. Oettinger in Proc. Symposia Appl. Math. XII. 104)