An overline or overbar. Sometimes numerals were written with both an overline and an underline. This had no particular meaning other than to emphasise the fact it was a Roman numeral. When using all capitals with a serif font the numerals look like they are both overlined and underlined.
When applied to an individual numeral, an overbar signifies the number be multiplied by 1000. If it is also underlined then it has no significance.
VINCULUM - The bar above a roman numeral
The bar in Roman Numerals is named vinculum. This bar is placed above a Roman Numeral and indicates a multiple of 1000. The Romans did not use this name for the bar, it was introduced in the Middle Ages.
A bar over a Roman numeral multiplies the base numeral by 1000. Example: V with a bar over it = 5000
It indicates multiplication by a thousand
The number 8000 can be represented by the Roman numeral VIII with a horizontal bar placed above it or by the Roman numeral [VIII]
VINCULUM - The bar above a roman numeral
The bar in Roman Numerals is named vinculum. This bar is placed above a Roman Numeral and indicates a multiple of 1000. The Romans did not use this name for the bar, it was introduced in the Middle Ages.
A bar over a Roman numeral multiplies the base numeral by 1000. Example: V with a bar over it = 5000
It indicates multiplication by a thousand
The number 8000 can be represented by the Roman numeral VIII with a horizontal bar placed above it or by the Roman numeral [VIII]
The number 100,000 can be represented in Roman numerals by the Roman numeral C with a horizontal bar placed above it or by the Roman numeral [C]
A roman numeral with a bar above it represents the same value multiplied by 1,000. For example, a bar above the letter X (X̅) would represent 10,000.
(VIII) or VMMM but with an horizontal bar above the numeral V
The number 5000 can be represented by the numeral V with a horizontal bar placed above it or by the numeral [V]
The number 1 billion can be written as the Roman numeral [M] with a horizontal bar above it (which I can't write with my keyboard).
20000=XX with horizontal bar above the 2X's.
25000=XXV with horizontal bar above XXV