Pair of homologous chromosomes is called bivalent or tetrad form at zygotene.
Zygotene or Zygonema or Synaptic stage:
The homologous chromosomes come to lie in pairs.
In each homologous pair of chromosomes, one chromosome comes from mother through ova and is called maternal chromosome and the other chromosome comes from father through sperm and is called a paternal chromosome.
This pairing of homologous chromosomes is called synapsis and these paired chromosomes are called bivalents.
Homologous chromosomes in a pair are of the same length, carry the same genes in the same sequence.
Electron micrographs of this stage indicate that the chromosome synapsis is accompanied by the formation of a complex, tripartite, proteinaceous structure called synaptonemal complex.
The number of bivalents in a cell is equal to the number of haploid chromosomes. The chromatids are still not visible.