parameter

a property that is a variable in general, but for the specific context in question is a constant.

Age is a variable. In other words, it is a score that varies from person to person, from measurement to measurement.

Average age is also a variable. Boston has an "average age", and so does Sydney, and so does the seventh grade science class at the local high school. However, for each of these populations (all people in Boston, all people in Sydney, all seventh grade science students at the local school) the value is fixed. If you're studying seventh grade science students, average age only has one number. As far as you are concerned, it is a constant.

If you are doing, say, health research on smoking in Sydney males, the average age is fixed. You treat it as if it is a constant. A value like this, that is treated as a constant, even though in theory it is a variable, is called a parameter.

Properties of a population, such as mean and standard deviation are often called population parameters.