Def or DEF may refer to:
Defamation—also calumny, vilification, and traducement—is the communication of a false statement that harms the reputation of an individual person, business, product, group, government, religion, or nation.
Deforestation, clearance or clearing is the removal of a forest or stand of trees where the land is thereafter converted to a non-forest use.
Defibrillation is a treatment for life-threatening cardiac dysrhythmias, specifically ventricular fibrillation (VF) and non-perfusing ventricular tachycardia (VT).
The Defenders is a set of fictional superhero groups with fluctuating membership appearing in American comic books published by Marvel Comics.
Def Leppard are an English rock band formed in 1977 in Sheffield as part of the new wave of British heavy metal movement.
Deflategate was a National Football League (NFL) controversy involving the allegation that the New England Patriots deliberately underinflated footballs used in their victory against the Indianapolis Colts in the American Football Conference (AFC) Championship Game of the 2014–15 NFL playoffs.
The Defence Research and Development Organisation (DRDO) is an agency of the Republic of India, charged with the military's research and development, headquartered in New Delhi, India.
Deformation in continuum mechanics is the transformation of a body from a reference configuration to a current configuration.