It is often called the thousands separator or digit group separator.
For ease of reading, numbers with many digits
before or after the decimal mark may be divided
into groups using a delimiter, with the counting
of groups starting from the decimal mark in both directions. This delimiter is usually called a "thousands separator", because the digits are
usually in groups of three, that is, thousands. The
most general name for this delimiter is "digit
group separator", because thousands are not
always the relevant group. For example, in various
countries (e.g., China, India, and Japan), there have been traditional conventions of grouping by
2 or 4 digits.
Some countries use a period for the separator and a comma for the decimal mark.