The decimal separator is a symbol used to mark the boundary between the integral and the fractional parts of a decimal numeral. Terms implying the symbol used are decimal point and decimal comma.
The decimal separator is mathematically a radix point.
The choice of symbol for the decimal separator affects the choice of symbol for the thousands separator. Consequently the latter is treated in this article as well.

History
Numbers with many digits before and/or after the decimal separator may be divided into groups of three, starting from the decimal separator in both directions.The symbol for this is called the thousands separator or, more generally (see India below), digit group separator. If the decimal separator is a point, the thousands separator is often a comma or a space. The latter is recommended in the SI/ISO 31-0[2]; when a space is used, it is often used after the decimal separator too, thus "1 234.567 89". If the decimal separator is a comma, the thousands separator is often a point or a space. Notations like "12,345", "12.345", "12,345.678", and "12.345,678" are ambiguous if the notational system is not known.
Making groups of three digits also emphasizes that there is a base 1000 of the numeral system that is being used. See Decimal superbase.
The house manuals of style for many publishing organizations state that thousands separators should not be used in normal text for numbers from 1000 to 9999 inclusive where no decimal fractional part is shown (in other words, for four-digit whole numbers). This does not apply in mathematical and other technical contexts.

Examples of use
Countries where a dot is used to mark the radix point include:
Australia, Brunei, Botswana, Canada (English-speaking), Hong Kong, India, Ireland, Israel, Japan, Korea (both North and South), Malaysia, Mexico, New Zealand, Nigeria, Pakistan, People's Republic of China, Philippines, Singapore, Sri Lanka, Taiwan, Thailand, United Kingdom, United States (including insular areas), Zimbabwe

Dot countries
Countries where a comma is used to mark the radix point include:
Albania, Andorra, Argentina, Austria, Azerbaijan, Belarus, Belgium, Bolivia, Bosnia and Herzegovina, Brazil, Bulgaria, Cameroon, Canada (French-speaking), Costa Rica, Croatia, Cuba, Chile, Colombia, Cyprus, Czech Republic, Denmark, Dominican Republic, Ecuador, El Salvador, Estonia, Faroes, Finland, France, Germany, Greece, Greenland, Guatemala, Honduras, Hungary, Indonesia, Iceland, Italy, Latvia, Lithuania, Luxembourg (uses both separators officially), Macedonia, Moldova, Netherlands, Norway, Nicaragua, Panama, Paraguay, Peru, Poland, Portugal, Romania, Russia, Serbia, Slovakia, South Africa (officially, but dot point is commonly used in business and science), Slovenia, Spain, Sweden, Switzerland, Turkey, Ukraine, Uruguay, Venezuela, Vietnam

Momayyez countries

Algorism
Radix point