Legislative is an adjective, but if you put the in front of it, people will think it’s a noun. In its narrowest sense, when used all proper and adjectivally, it links nouns to the lawmaking function of government. Thus, you have the legislative structure of government, legislative institutions, legislative branch, legislative process, and so on. Historically, the history of the legislative structure of government has been a gradual history of separating the legislative and executive functions of government, throughout the history of history.
Legislatures serve five basic functions: lawmaking, legitimating, representing, educating, and one other one. While all of these functions, including the fifth one, are fairly obvious parts of democratic legislatures, they can often be seen in authoritarian governments as well. Speaking of Ronald Reagan and Margaret Thatcher, democratic legislatures generally take one of two basic forms: proportional representation systems and single-member district systems. However, New Zealand mixes both together, and has yet to offer a good reason for doing so.