Supposition (noun)

1. An assumption or hypothesis that is taken to be true without proof.
2. The act or process of assuming or hypothesizing.

Origin:

Late middle english (denoting a thing supposed): from old french, or from latin suppositio(n-), from supponere "to place under" (see suppose).

Examples:

1. His theory is based on the supposition that the earth is flat.
2. The supposition that the universe began with a Big Bang is widely accepted by scientists.
3. The detective's supposition that the murder weapon was a knife turned out to be correct.
4. The supposition that the company would go bankrupt was proven to be false.
5. The study's results are based on the supposition that there is a correlation between obesity and diabetes.