What are the individual models within a machine learning ensemble called? I am aware that an ensemble machine learning model is a stack of two or more machine learning models. Is there a word to refer to those individual models that go into the ensemble model? (i.e. a specific term/jargon?) 

I wish to refer to them and am not aware of any specific terminology to describe them. I've considered: 


"sub-models" 
"component models" 
even "pancake models" (since people
generally understand what's being referring to when they hear 'one of the
pancakes in the model')
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.