A dry run is most simply defined as practice, rehearsal, or a trial for an event. In many instances, it refers to practice before a performance, that goes through all of the actions but with one missing element. The term is also commonly used in military procedures, and the same basic principle applies in that scenario. The origins of this idiom are not completely clear, but it may have started with fire departments who demonstrated their skills at public functions in the late 19th century.
For most people, the term dry run is synonymous with a dress rehearsal or trial. For example, the dress rehearsal for a play goes through all of the motions of the play, including sets, costumes, and staging but is missing the audience, making it a dry run. This can also apply to a trial run of an experiment in which most of the actions of the experiment are performed but the entire procedure is not executed. Another meaning refers to emergency services that receive a call and respond to it, but give no actual aid because the call becomes invalid before they arrive. In this instance, the emergency personnel are going through all the motions of a call without the element of following through and giving aid.
Military settings are another place where the term dry run is used and has its own specific meaning. Military personnel sometimes have practice drills. During these learning exercises, the use of actual ammunition would obviously be rather hazardous, so they are performed without using any real ammunition. This is practice for a real event that is missing one key element and is therefore referred to as a dry run.
In the late 19th century, it was common for fire departments and hose companies to put on performances, that showcased their hose running and fire fighting skills, at fairs or other public events. For some of these demonstrations, fire fighters would run hoses through a course or up ladders and act as if they were putting out a fire but without using any water, hence the name, dry run. Conversely, these same drills would sometimes be performed with water and were then referred to as a wet runs.