Definition
 

I am a US pilot and have read many post refrencing TRE and TRI.
What exactly are TRE/TRI?

Thank you

Type Rate Examiner/Instructor

I think you would know them as "checkies".

definition
 

So TRE and TRI refer to Check Airman equivalent in the USA.

Thank you.

A TRI is a Type Rating Instructor and carries out conversion training. The TRE (Examiner) is also a TRI but has had examiner priviledges added. He can therefore carry out the end of conversion type rating test, or Licence Skill Test (LST) and subsequent annual Licence Proficiency Checks (LPC). He also carries out the company, or Operator, proficiency checks (OPC).

So, I guess the TRE is the "check and trainer" whereas the TRI is just the "trainer"