basis

(noun) a relation that provides the foundation for something. Synonyms: footing, ground. Examples: "They were on a friendly footing." "He worked on an interim basis."

(noun) the fundamental assumptions from which something is begun or developed or calculated or explained. Synonyms: base, foundation, fundament, groundwork, cornerstone. Examples: "The whole argument rested on a basis of conjecture."

(noun) the most important or necessary part of something. Synonyms: base. Examples: "The basis of this drink is orange juice."