either, the practice of being in a committed relationship with only one person at a time, such as being married to one person at a time; or, the practice of only having one sexual partner at a time.

People who are non-monogamous may be polyamorous, have open relationships, invite other people to join them in their sexual activity, and/or engage in sexual activity outside of the relationship on a recreational basis: though it should be noted, that typically, such relationships work on an open basis, and should not be confused with infidelity.