Chromatid

A chromatid is one of the two identical copies of DNA making up a duplicated chromosome, which are joined at their centromeres, for the process of cell division (mitosis or meiosis). They are called sister chromatids so long as they are joined by the centromeres. When they separate (during anaphase of mitosis and anaphase 2 of meiosis), the strands are called daughter chromosomes.

In other words, a chromatid is "one-half of a replicated chromosome". It should not be confused with the ploidy of an organism, which is the number of homologous versions of a chromosome.

Dyad
A dyad is a pair of sister chromatids. This occurs in prophase I of meiosis. After DNA replication, the two sister chromatids align side-by-side and appear to have an undivided centromere, in contrast to mitosis, in which each chromatid appears to have its own separate centromere.

The term dyad can also be used to describe protein morphology.