Absolute dating uses properties
Radioactive isotopes decay at a known rate, and have a predictable half life; the time it will take for half of a given quantity of radioactive isotope to decay into a stable state.
Using these known numbers, it is possible to estimate the relative age of an object..
Different radioactive isotopes are useful for measuring different time scales, but not all are present in any given object (ie- different minerals or rocks)..
This method is not without issue, and estimates that are grossly inaccurate can occur, either by an error in execution of the methods, or because of incorrect assumptions of the original condition/quantity of component materials..
It is also possible that rates of radioactive decay may not always be constant; the scale of time in which scientists have been able to consistently measure these rates has not been sufficient to confirm or deny this prediction.
Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy.
Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events.
In archaeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates (coins and written history).
Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped-charge dating methods such as thermoluminescence dating of glazed ceramics.
Coins found in excavations may have their production date written on them, or there may be written records describing the coin and when it was used, allowing the site to be associated with a particular calendar year.