What are the individual models within a machine learning ensemble called?
I am not aware of a specific definition. Wikipedia does not mention such a term either. I would prefer "components", "individual/constituent models", or something like that.

If you definitely want to find a "correct" term, one way to discover it (if it exists) is to look into an ensemble learning early paper. 
To arrive at one, a good way is to search for a survey paper (e.g. here) and go on a journey to the past to find that original study.

From a quick skim of the above, you might want to look at Exploratory Data Analysis (Tukey 77), if you manage to find the pdf.