Basis is the foundation or main reason for something, the underlying support that makes other things possible. When a teacher grades your work on the basis of effort and improvement, those are the key factors influencing your grade. When scientists accept a theory on the basis of evidence, they're relying on facts and observations to support their conclusions.

Think of basis like the foundation of a house: everything else rests on it. Your friendship with someone might be built on the basis of shared interests and trust. A doctor makes medical decisions on the basis of symptoms, test results, and medical knowledge. When a coach chooses team captains on the basis of leadership skills, that quality becomes the determining factor.

The word often appears in the phrase on a regular basis, which simply means repeatedly or consistently. If you practice piano on a daily basis, you practice every day. If your family goes hiking on a weekly basis, you go each week.

Basis suggests something fundamental and necessary. When you say honesty is the basis of a good friendship, you mean it's essential, the core element without which real friendship can't exist.