'Harmful' definitions:

Definition of 'harmful'

From: WordNet
adjective
Causing or capable of causing harm; "too much sun is harmful to the skin"; "harmful effects of smoking" [ant: harmless]

Definition of 'Harmful'

From: GCIDE
  • Harmful \Harm"ful\ (h[aum]rm"f[.u]l), a. Full of harm; injurious; hurtful; mischievous. " Most harmful hazards." --Strype. --{Harm"ful*ly}, adv. -- {Harm"ful*ness}, n. [1913 Webster]