As a Noun:
1. The study of the nature and behaviour of natural things and the knowledge that we obtain about them. English = science.
2. An expert who does work in one of the sciences. English = scientist.
As a Verb or Adjective:
1. To relate to science or to a particular science. English = (be) scientific.
2. To do something in a systematic way, using experiments and tests. English = (be) scientific.