Generally, a null result is a result which is null (nothing): that is, the absence of an observable result. In science, it refers to an experimental outcome which does not show an otherwise expected effect. The term is a translation of the scientific Latin nullus resultarum, roughly meaning "none as a consequence".
Statistics
In statistics, specifically, a null result occurs when there are non-significant differences between experimental and control conditions. While some differences may in fact be observed, they are below the threshold set prior to testing. The cutoff for these significance values varies, but is often .05. This is considered evidence for the null hypothesis.
Physics
In physics, the results of the Michelson-Morley experiment, which did not detect the aether, was of this type. This experiment's famous failed detection, commonly referred to as the null result, caused the aether theory to be abandoned. Albert Einstein's theory of relativity, which followed close upon this null result, does not make use of the aether concept.
Logic
In logic, a null result is an invalid value of a proposition. It is one of the possibilities in a three-valued logic (true, false, or null result).
Law
In law, a null result can mean something lacking any legal or binding force.