Unstress

The breve-like unstress mark tells the performer not to emphasize a note where metre or phrasing would normally suggest stress. It is the counterpart to Schoenberg's stress mark, not a slur or fermata fragment.

Family: Articulation
Where it sits: Above the note
What it looks like: A curve or arc

Do not confuse it with

Stress adds intended emphasis; unstress suppresses an expected one.

What the sources say

Dorico Pro — Articulations Steinberg
Indicate notes that should be emphasized or not emphasized where that contradicts the prevailing meter, using stressed and unstressed marks.
music21.articulations — Unstress music21 / Massachusetts Institute of Technology
An articulation indicating lack of stress. Played a little quieter.
SMuFL — Articulation W3C Music Notation Community Group
articUnstressAbove — Unstress above

All notation signs