Step-in is a compound word that refers to the action of entering or intervening in something. There are several synonyms that can be used to express the same meaning as step-in, such as intervene, interfere, intrude, meddle, interrupt, interpose, and mediate. Each of these words conveys the idea of stepping into or getting involved in an existing situation or discussion. For example, you might use the word intervene to describe a situation where you enter to try and resolve a dispute between two parties. Interfere might be used to describe a well-intentioned act that ultimately disrupts a plan or outcome. The precise word to use will depend on the context and intended meaning.