What is the meaning of basis?

noun

• A relation that provides the foundation for something.
  • usage: "they were on a friendly footing"; "he worked on an interim basis"
  • synonyms: footing, ground
• The fundamental assumptions from which something is begun or developed or calculated or explained.
  • usage: "the whole argument rested on a basis of conjecture"
  • synonyms: base, foundation, fundament, groundwork, cornerstone
• The most important or necessary part of something.
  • usage: "the basis of this drink is orange juice"
  • synonyms: base