Typically, the term "open relationship" describes a committed relationship between two individuals that is not strictly monogamous. To "open" a relationship, the partners need to discuss when and how they allow romantic experiences with others.
Individuals in an open relationship still share a significant level of commitment to one another just as individuals involved in a traditional, monogamous relationship do. The key difference with an open relationship is an understanding between the partners that sexual experiences with others can, at times, be acceptable. In some open relationships, potential sexual encounters with outsiders need to be discussed and "authorized" by the other person; however, this is not always a requirement.