Basis (noun)

1. The underlying foundation or support.
2. The principal or underlying substance or material.
3. The fundamental or essential idea, principle, or factor.
4. A point of departure or starting point.
5. The amount or quantity on which a calculation or charge is based.

Origin:

Late middle english, from latin basis foundation, from greek ba·sis foundation, step.

Examples:

1. The basis of their argument was flawed.
2. She tried to find the basis for the strange noise.
3. The basis for their relationship was mutual respect.
4. The report was based on a sample of data and may not be representative of the whole population.
5. The stock price is determined on a supply and demand basis.