What is a tetrad?

During prophase I of meiosis, the members of each pair of homologous chromosomes move together and come to lie side by side. At this time, the twin cromatids of each chromosome are held together by a pair of long, thin protein axes that run along their entire length. As a result, the two chromatids are held together so tightly that they are indistinguishable. The protein axes of the two homologous chromosomes now join by a means of protein cross-bridges to form a compound structure, which lines up the four chromatids - often referred to as a tetrad - in perfect alignment. This pairing is known as synapsis.