As a Noun:
1. The thing that a single unit of a language (word or sign) or a whole expression refers to or expresses. Often this 'thing' is just other words and signs, rather than something you can point at. English = meaning, significance.
2. The study of the meaning of words, signs, and sentences. English = semantics.
As a Verb or Adjective:
1. Of a unit of language (word or sign) or a whole expression, to refer to a thing or idea or express something. English = mean. Formal English = signify.
2. In the area of language, anything that relates to meaning. English = semantic.