Complex sentence: There is a certain leeway in using the term "classical group" depending on the context, but the uniting feature of classical Lie groups is that they are close to the isometry groups of a certain bilinear or sesquilinear forms.
Split into simpler sentences: There is a certain leeway in using the term "classical group" depending on the context. The uniting feature of classical Lie groups is that they are close to the isometry groups of a certain bilinear or sesquilinear forms.