Common words are sometimes given specialized meanings in a certain profession. Examples are the distinctions between “accurate” and “precise,” between “engine” and “motor,” and between “font” and “typeface.”
Sometimes the technical distinctions preserve original meanings, but in other cases they narrow the range of original meanings.
In any case, in non-technical contexts these distinctions are not preserved. Whether or not this is seen as a mistake depends on the “usage community” uttering or receiving the language involved.
Theologians distinguish between “dogma” and “doctrine,” but they do not have a license to prevent journalists and others from using these words interchangeably in non-theological contexts.
I wish lawyers had the ability to restrain other writers from using “actionable” to mean “doable,” but the fact is that a new meaning for the word has evolved and passed into common usage by people who have no awareness of the word’s original meaning.
“Libel” has traditionally been defined in law as written defamation and “slander” as spoken; but the distinction is seldom preserved outside of legal contexts.
Most dictionaries concede the point by listing one or more supplementary definitions like these from the New Oxford American Dictionary, 2nd edition (The Apple Dictionary):
• a false and malicious statement about a person.
• a thing or circumstance that brings undeserved discredit on a person by misrepresentation.
“Libel” is probably less often used of spoken defamation than “slander” is used to refer to written defamation. “Slander” is a more common word than “libel,” and many people consider libel to be a form of slander.
Legal professionals and scholars may insist on the distinction (see the relevant article in Wikipedia), but they cannot overrule standard non-technical usage. Dictionaries reflect that fact.