The second-person singular is a grammatical term used to refer to the pronoun or verb form that specifically addresses one individual. However, there are several synonymous terms that can be used instead of the second-person singular. One of these terms is the "informal singular," which typically refers to the use of the singular pronoun 'you' in casual or informal settings. Another term that is often used in place of second-person singular is the "singular you," which similarly refers to the use of the pronoun 'you' when speaking to one individual. Other synonymous terms include the "singular form," "individual form," and "personal form.