As much as they like
Most pieces of legislation have a “dictionary” detailing, for the purposes of that legislation (or generally) what specific words and phrases mean. This can broaden (or narrow) the definition compared to how they are used in normal English.
The purpose of this is not to set a trap for the unwary, although this may happen, but to introduce precision and to allow a short defined term to be used in the drafting rather than having to explain what is meant verbosely every time it’s used.
Of course, they can’t redefine terms so that they give themselves jurisdiction when they otherwise wouldn’t have it. For example, in australia, the Constitution gives the Federal Parliament the power to make laws about, among other things, “external affairs”. A law that tried to define “external affairs” more broadly than the Constitution does (which it doesn’t, so we fall back on what it means in English) would be invalid.