There are various synonyms for the phrase "sign in," which can be useful in different contexts. One common synonym is "log in," which refers to the process of accessing an online account by providing login credentials such as a username and password. Another synonym is "check in," which is often used in physical locations like hotels or airports and refers to the process of registering one's arrival. "Authenticate," "verify," and "confirm" are other synonyms that are used to denote the act of providing evidence or proof of identity or membership. Overall, using different synonyms for "sign in" can help communicate more clearly and avoid repetitive language.