A polarity chron, or chron, is the time interval between polarity reversals of Earth's magnetic field. It is a term used in stratigraphy (a branch of geology) to name the time interval represented by a magnetostratigraphic polarity unit. It represents a certain time period in geologic history where the Earth's magnetic field was in predominately a "normal" or "reversed" position. Chrons are numbered in order starting from today and increasing in number into the past. As well as a number, each chron is divided into two parts, labelled "n" and "r", thereby showing the position of the field's polarity. A chron is the time equivalent to a chronozone or a polarity zone.
It is called a "polarity subchron" when the interval is less than 200,000 years long.