Betrayal
Etymology
From "betray," from be- plus Old French trair, from Latin tradere "to hand over" (trans- "across" plus dare "to give"). To betray is literally to hand someone over — the same root gives us "tradition," what is handed down. Betrayal is a handing-over turned against the one who trusted you.
noun · /bih-TRAY-uhl/
The breaking of trust by someone who was supposed to be loyal to you.
What it names
A handle for the specific wound that only the trusted can inflict — harm made worse precisely because it came from inside the circle of faith.
Example
It was not the affair itself but the months of lying that felt like the real betrayal.
Related idea
Distinct from perfidy: perfidy names the trait of habitual, calculated treachery — a quality of character — whereas betrayal names the act and the wound from the victim’s side. One perfidious person can commit many betrayals; a single betrayal need not make someone perfidious.