A Telomere is a part of the human chromosome. Doctor. Jennifer Keller has said that each time a cell divides, the chromosome is torn apart and rebuilt, so in the process, a small amount of the telomere gets used up. As a result, telomeres become shorter over time. The more humans age, the shorter they get.
When the First Atlantis Reconnaissance Team discovered Dr. Carson Beckett on one of Michael Kenmore's outposts, since he died a year previously, they wondered who he is. Keller discovered that his telomeres were 30% shorter than they should be for a man of his age, meaning that his original DNA came from an adult cell; one where the telomeres had already started to degrade, proving Beckett as a clone. (SGA: "The Kindred, Part 2")