Final answer:
The difference between synapsis and a chiasma in meiosis is that synapsis is the physical pairing of two homologous chromosomes, while achiasmata is the x-shaped structure formed during crossing over.
Step-by-step explanation:
The difference between synapsis and a chiasma in meiosis is that a synapsis is the physical pairing of two homologous chromosomes during prophase I, forming a tight alignment of genes on the chromatids. On the other hand, a chiasma is the x-shaped structure that is formed when crossing over occurs between the non-sister chromatids of homologous chromosomes, leading to an exchange of DNA segments. The chiasma remains until anaphase I and is responsible for genetic recombination and the creation of genetic variation.