Base vs. Basis

Base

• Foundation (literal or figurative): The bottom part of something; a starting point.
  • The base of the statue.
  • Her argument lacked a factual base.
• Main ingredient / component:
  • The sauce has a tomato base.
• Military installation:
  • An air force base.

Basis

• Fundamental reason or principle: The underlying justification or groundwork for an idea, decision, or system. Almost always abstract.
  • What is the basis for your claim?
  • They met on a regular basis.
  • The decision was made on the basis of strong evidence.

Key difference: “Base” can be concrete (the bottom) or a starting point/foundation. “Basis” refers almost exclusively to the reason or principle upon which something is founded or done.