爸 (bà) is an informal word for father or dad, equivalent to saying dad rather than father in English. It is commonly used alone as a form of address (爸！) or with a possessive (我爸, my dad). The more formal or compound form is 爸爸 (bà ba), and the most formal term is 父亲 (fù qīn). 爸 on its own feels slightly more clipped and is especially common in northern Chinese speech.