There are several different definitions of an open relationship. From a non-sexual standpoint, an open relationship is where both partners hide nothing from each other, including their past relationships and sexual experiences. However from a sexual standpoint, an open relationship signifies a relationship where both partners have agreed that monogamy, sexual fidelity or exclusivity are not expected.
Some people equate an open relationship with polyamory. Although the two might define a similar interaction where everyone involved can date or have a relationship with other people, polyamory focuses more on the experience of romantically loving more than one person at a time, as opposed to an open relationship which merely refers to people being committed to one another yet non-exclusive. Open relationships differ from no strings attached relationships, in that there is no commitment with a no strings attached arrangement. A friends with benefits relationship also differs from an open relationship, although the two terms can describe the same relationship. Whereas friends with benefits offer sexual companionship between two friends, it doesn't imply a level of commitment or exclusivity.