A more thorough explanation:

Pellex

In ancient Rome, a pellex was a woman who lived with a man as his lover without being married to him. She was considered a concubine.

Julius Caesar had a pellex named Servilia, who was also the mother of one of his enemies, Brutus. Another famous pellex was Clodia, who was involved in a scandalous affair with the poet Catullus.

These examples show that a pellex was a woman who had a romantic relationship with a man but was not his wife. In ancient Rome, this was not considered socially acceptable, but it was still a common practice among the upper classes.