The word partner has been officially preferred over the word husband, wife, or spouse, for the last few decades. But a casual partner definition involves even less commitment than that.
The possible synonyms are hooksexup mate, travel mate, sugar dating mate, etc. It means, a casual partner is equal to a mate, or a friend with benefits, rather than to a responsible companion.
However, the responsibilities of their own kind do exist in no-strings-attached affairs and relationships. First of all, it’s the duty to respect another person’s freedoms and personal space.
When talking about the unplanned one-time sex, another person is called a partner only in a medical meaning, as the analogue to a sexual partner medical questionnaires ask us about.
While in casual dating blogs and between the participants themselves, a hookup partner is someone who we meet once or twice a week, but rather regularly and definitely more than once.
In this case, the interaction differs from the serious one only with the absence of emotional attachment and expectations from the future. This is how the word casual partner is understood.