◌̅

A horizontal line to be placed above letters and digits. It connects to itself on either side if it's also placed over those letters.

◌̅

1. Used in some languages to mark abbreviations.

   Ι̅Σ̅ Χ̅Σ̅, a Greek abbreviation of Jesus Christ.

◌̅

1. (mathematics) Drawn over repeating digits.

   12 ÷ 7 = 1.7̅1̅4̅2̅8̅5̅ = 1.714285714285714285...

2. (geometry) Drawn over letters which represent a line segment.

   segment A̅B̅ has the same length as segment C̅D̅

3. (physics) Used to denote antiparticles.

   p (proton); p̅ (antiproton)

4. (mathematics) Indicates the complex conjugate of a complex number.

   z̅ = x̅+̅i̅y̅ = x−iy

5. (statistics) Used to mark the mean of a sample.
6. (Roman numerals) Multiplies the value of a group of digits by one thousand.

   IV — four; I̅V̅ — four thousand; I̿V̿ — four million

7. (Greek numerals) In Byzantine Greek and in Coptic, converts a letter into a digit. E.g. κ̅ (modern κʹ) '20'.
8. (phonetics) marks a string of syllables or segments with high pitch. Compare ˹ ... ˺.
9. (actuarial notation) Used to indicate continuous payments.

   a̅_n̅| ― n-year annuity payable continuously

• ◌̄ (“macron, a diacritic over a single letter that does not join to itself.”)

• Roman numerals on Wikipedia.Wikipedia