A parliament has two distinctive characteristics. First, it is a legislature. What this means is that it is the body of government that makes the laws for a country. Second, the word "parliament" refers to a particular kind of a legislature. A parliament is found in systems of government in which there is little separation of powers. In a parliamentary system, there is no separate executive branch (like that headed by the President of the United States). Instead, the chief executive (as in the Prime Minister of the United Kingdom) is actually a member of the legislative branch.
So, the word "parliament" is used to refer to the body that makes the laws for a country that uses a parliamentary system.