Chromatids
Sister chromatids are 2 identical strands of DNA, held together at the Centromere after being copied in the S-Phase of the Cell cycle [1]. During Mitosis chromatids separate, each entering identical daughter cells.
Dyad
A dyad[2] is a sister pair of chromatids. These are generated in prophase 1 of meiosis. The two sisters line up side by side and share a centromere.