Medical Definition of COHESION. 1. : the act or process of sticking together tightly. 2. : the molecular attraction by which the particles of a body are united throughout the mass—compare adhesion 3.ull Definition of COHESION. 1. 2. : union between similar plant parts or organs. 3. : molecular attraction by which the particles of a body are united throughout the mass. — co·he·sion·less \-ləs\ adjective. See cohesion defined for English-language learners. See cohesion defined for kids. ADVERTISEMENT.