noun (n)

• a relation that provides the foundation for something(noun.linkdef)

  Example:
  He worked on an interim basis.
  

  Synonym:
  footing, ground
  

  source: wordnet30
• the fundamental assumptions from which something is begun or developed or calculated or explained(noun.cognition)

  Example:
  The whole argument rested on a basis of conjecture.
  

  Synonym:
  base, cornerstone, foundation, fundament, groundwork
  

  source: wordnet30
• the most important or necessary part of something(noun.linkdef)

  Example:
  The basis of this drink is orange juice.
  

  Synonym:
  base
  

  source: wordnet30
• The foundation of anything; that on which a thing rests.(noun)

  source: webster1913