The term "main part" refers to a significant or principal component of something. There are various synonyms for this term that can be used depending on the context of the sentence. Some examples include "core," "crux," "essence," "foundation," "heart," "nucleus," "pillar," "substance," and "centerpiece." These synonyms can be employed interchangeably in various texts to provide diverse expressions and prevent redundancy. For instance, instead of continuously using "main part" in one text, other synonyms can be incorporated to enhance the flow of the sentence, making the content more interesting and unique.