What is the definition of passa ? The answer on this question mentions passa in the comments. 

Having never encountered the term in my studies, what is an overview of the term? What school of Buddhism does it originate from?
As per my dictionaries: 

Phassa (sanskr. sparça): contact, touch, tangibility, tactile sensation, a momentary union of the sense-object, sense-door, and sense-consciousness.

It's a standard term coming all the way back from the Pali Canon and used widely by Theravada and Abhidhamma.