Formal:
* Feline: This is the most formal and scientific term.
* Catidae: This is the scientific family name for cats.
Informal:
* Kitty: This is a common and affectionate term.
* Pussycat: This can be affectionate or playful, but can also be considered outdated.
* Moggy: This is a British term for a domestic cat, often used playfully.
* Tabby: This refers to a cat with a striped coat.
Other:
* Frisk: This is a more archaic term for a cat, often used in poetry.
* Grimalkin: This is an old-fashioned term for an old cat, often used with a negative connotation.
The best synonym depends on the specific situation and tone you're trying to convey.