A one to one relationship is a relationship between two people without third party factors. This is the typical interaction between two individuals, personal, social or professional. In most cases one to one relationships are on varying levels, with no particular inference of a sexual relationship or any other qualifiers. This is the 'neuter' form of relationship. In common usage, it's used to describe a social situation between people, usually identifying the parties.

Database usage: A single entity's relationship, like an account number, or personal identification.