The word Casanova is generally used to refer to a man who is sexually promiscuous, and according to Merriam-Webster, an "unscrupulous lover." The original Casanova was a real person — Giovanni Giacomo Casanova — who was an 18th century adventurer, spy and writer who developed a reputation as a seducer of many women.
In today's world, a Casanova might be called a playboy, ladies' man or womanizer. In modern pop culture, the term is often considered to be on par with the term "player" as this term describes a person with low morals who has little regard for the feelings of the opposite sex. Notably, the original Casanova once aimed to become a priest, but was expelled for bad behavior, serving for a time as a cardinal in the Roman Catholic Church.