A chromatid is one of the two strands that form when a chromosome replicates itself for the process of cell division (either mitosis or meiosis). Prior to replication, human somatic cells contain 46 chromosomes. Thus, after replication, there would be 92 chromatids.
In other words, a chromatid is one half of a replicated chromosome. Two chromatids are joined together by a central centromere to form one single chromosome. The two chromatids that are joined by a centromere are sometimes referred to as “sister chromatids.” Chromatids remain as long as they are attached to the centromere. Thus, chromatids are present during prophase and metaphase of mitosis.
Once a chromatid becomes detached from the centromere, it becomes a chromosome. Thus, it is during anaphase of mitosis that the separation of sister chromatids occurs.