Digamy is a second marriage after the death or divorce of a previous spouse.
World Wide Words subscriber Tim Russell wrote to me to wonder why this word was not more often used, since many of us are digamists without knowing that we are, and pointing out that it describes a common and entirely acceptable relationship. Perhaps those few people who have come across it believe it is just another term for bigamy (a sense the word could indeed once have had) or perhaps the ease with which the two words can be confused led to the less common one dropping out of casual use. It does appear on rare occasions in academic works.
Digamy comes directly from Latin digamia, with the same sense, which in turn derives from Greek; its first recorded appearance was in 1635. An alternative that once had the same sense is deuterogamy, though this is now equally defunct.