In only good humor, I wanted to point out 
something. you do realize you posted asking what 
it was, and then argued with someone when they 
told you what they thought it was? If you don't 
know what it is, why are ya arguing? Just my 
thought. 
"Cel" just stands for "celluloid" right? I assume 
calling if it's plastic you can call it a cel and 
not be lying. As long as you don't say it's 
a "production cel" I don't think anyone will look 
down on you.