The custom or practice of having one mate at a time. Each relationship may be serious and committed, or not. The term is used in contrast with strict "monogamy", which can imply just one partner over a lifetime.
People who remarry (and who have exclusive relationships, and who don't cheat) are practicing serial monogamy.
A practice by which a person experiences a large number of short-lived romantic relationships or "flings." Those engaged in this practice probably have had numerous sexual partners but have not engaged in cheating or infideloutous behavior.
People engaged in serial monogamy use it as a tool to avoid marriage and any other long term commitments and could be labeled "commitment phobes."