Janus words are words with two opposite meanings. Such words are named after the Roman god Janus who has two faces that look in opposite directions.
Examples of Janus words are:
"sanction," which means approved by an authority, and otherwise condemned;
"cleave," which can mean to cling to or to separate;
"screen," which can mean to review or display or to hide or shield from view, and
"trim," which can mean to remove things or add things.
Source: Grammar Girl