Which prefix refers to the concept of "joined together"?

The prefix "sym-" is derived from the Greek word "syn," which means "together" or "with." It conveys the idea of things being joined together or occurring together. In medical terminology, "sym-" is often used in words that describe conditions or actions involving a union or combination. For example, the term "symbiosis" refers to a close and often long-term interaction between different biological species, essentially highlighting the concept of living things that are joined or associated with one another.

The other prefixes do not convey the same meaning. "Para-" typically means "beside" or "near." "Circum-" refers to something that is "around" or "about," indicating encirclement rather than a joining. "Post-" means "after," indicating a position in time rather than a relationship of unity. Thus, "sym-" is the most appropriate prefix when referring to the concept of being "joined together."