Imperial Germans is the common translation of the German word Reichsdeutsche (adj. reichsdeutsch). It refers to German citizens, and by the word sense means people coming from the German Empire, i.e. Imperial Germany or Deutsches Reich, between 1871 and 1945 (and later when referring historically to these times).
The key problem with the terms reichsdeutsch, volksdeutsch, auslandsdeutsch, and related ones is that the usage of the words often depends on context, i.e. who uses them where and when. There are, in that sense, no general legal or "right" definitions, although during the 20th century, all terms acquired legal — yet also changing — definitions.
The reason for the differentiation is that there has been a historical shift in the meaning of what belonging to a nation means. Until the 19th century, a term such as German was not too meaningful, although the concept certainly existed. If anything, it was more seen as a cultural concept (including language, religion (in different forms), and already sometimes race in a vague sense). Only with the 1871 unification of Germany under Prussian leadership did some concepts acquire a legal-political meaning, which they have retained until now.
In a second meaning, for someone considering themselves German but living abroad, Imperial German means any German who is a citizen of Germany, as opposed to someone living abroad (and usually without a German passport). Part of the identity of ethnic German minorities living abroad — a classic example are the Baltic Germans — was to define themselves as German, using the pre-1871 concept. However, Imperial Germans visiting the Baltic provinces in the late 19th Century, for instance, resented the claims of the Baltic Germans to be German — for the Germans from Germany, to be German meant to be a citizen of the Reich, while for the Baltic Germans, it meant cultural-historical belonging. Today, when referring to the present, Germans from Germany usually only employ the term Imperial German within such a discourse (Germans working in Latvia talking to a Baltic German visiting there might, for example, refer to themselves as Imperial Germans).
The opposite of Imperial Germans is, then, depending on context and historical period, Volksdeutsche, Auslandsdeutsche, or a more specific term denoting the area of settlement, such as Baltic Germans or Russian Germans (Wolgadeutsche).