Can it be called an affair if it was just a one night stand?
When most people marry, there is a mutual understanding (usually in the form of a vow) that certain connections outside the marriage relationship will be out of bounds. This includes both physical and emotional connections. When one spouse violates these boundaries, they break their promise and betray their spouse.
Once you become intimately involved with another person (whether emotionally or physically or both... whether one time or many times... whether for a short time or a long time), you have done damage to the underlying trust in your marriage. Your definition of an affair might be descriptive of an ongoing relationship instead of a brief encounter, but now is not the time to debate word meanings. If your spouse calls it an affair, deal with it like an affair and work toward healing.
