A trial is, in the most general sense, a test, usually a test to see whether something does or does not meet a given standard.
In law, a trial is the presentation of information in a formal setting, usually a court, with the object of determining whether or not a person (or other legal entity such as a corporation) has broken a law. See, for example: jury trial, trial by ordeal.
In science, a trial is the result of a given run of a given experiment, with the usual object of testing a scientific hypothesis. Multiple trials are usually run, when possible, for an experiment, in order to offset the effects of random error.
In proprietary computer software (and some other commercial products), a trial version or trial or piece of trialware is a (usually gratis and FRS) version of a product that only works for a limited period of time (and sometimes has reduced functionality). A trial version of a software package (or piece of trialware) is often produced by a software publisher to help users get a feel for it before deciding whether to buy the full version . See the more general term, shareware.
In probability mathematics, a trial is an action that results in one of a number of outcomes or elementary events.