noun
1. (Law) Law
a. defamation in some transient form, as by spoken words, gestures, etc.
b. a slanderous statement, etc.
2. any false or defamatory words spoken about a person; calumny
vb
to utter or circulate slander (about)
[via Anglo-French from Old French escandle, from Late Latin scandalum a cause of offence; see scandal]
As a layperson's term it's false speech that harms the reputation of the person(s) being talked about.
As a crime or civil offense (grounds for a lawsuit) it is defined in the laws of the jurisdiction where it is alleged to have occurred. You need to look at those laws for the exact definition that will determine whether the crime or civil offense of slander has occurred.
In the U.S., because of First Amendment protection of free speech, certain public figures cannot sue as easily for slander as private individuals can.