By asking if slander is 'illegal', do you mean criminally?? If so, in most jurisdictions, slander is not considered a criminal act, but is a civil wrong.
'Slander' is defined as follows by 'Merriam-Webster's Dictionary of Law':
slander; Anglo-French esclandre, from Old French escandle esclandre scandal, from Late Latin scandalum moral stumbling block, disgrace, from Greek skandalon, literally, snare, trap
1: defamation of a person by unprivileged oral communication made to a third party
also : defamatory oral statements
2: the tort of oral defamation