As a noun, the term is always used in the plural, even when it refers to a single event: if a judge fines a party, it is not said that he/she imposed a sanction, but that he/she imposed sanctions.
A judge may sanction a party during a legal proceeding, by which it is meant that he/she imposes penalties.
Conversely, the word may be used to mean "approve of," especially in an official sense. "The law sanctions such behavior" would mean that the behavior spoken of enjoys the specific approval of law.