V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

RTI
       ReTurn from Interrupt


The Jargon File (version 4.4.7, 29 Dec 2003):

RTI
 /R?T?I/, interj.

    The mnemonic for the ?return from interrupt? instruction on many computers
    including the 6502 and 6800. The variant RETI is found among Z80 hackers.
    Equivalent to ?Now, where was I?? or used to end a conversational
    digression. See pop.


The Free On-line Dictionary of Computing (30 December 2018):

RTI
RETI

   Return from interrupt