The law recognizes that the verbal, oral or libel variety of defamation can be more damaging than slander.
In Gatley on Libel and Slander:
"Libel is committed when defamatory matter is published in permanent form or in a form which is deemed to be permanent. Defamation published by spoken word or in some other transitory form is slander."
In Pollard v Lyon, the US Supreme Court (1875, cited at 91 US 225), adopted these words:
"(Defamation), in writing or in print, says the commentator, has always been considered in our law a graver and more serious wrong and injury than slander by word of the mouth, inasmuch as it is accompanied by greater coolness and deliberation, indicates greater malice, and is in general propagated wider and farther than oral slander."
Historically, the common law used the word to capture both libel and slander, which lawyers today prefer to use the word defamation in that context. The common law described what we now call slander as verbal or oral slander and libel as writtten or printed slander.
REFERENCES:
Milmo, P. and others, Gatley on Libel and Slander (London: Seeet and Maxwell, 2004).