A WDAY television viewer recently inquired about the difference between a rain shower and just plain rain. The classic meteorological definition is that a shower is brief and/or intermittent whereas rain is steady. However, there is no specific time limit for when a shower crosses the threshold and becomes just rain.
For most of us in the business of forecasting, a shower differs from rain by being convective in nature. What we call showers are caused by smaller scale updrafts of air and so are briefer or more intermittent than a general area of rain which is caused by a general, gradual rising motion over a large area.
In general, if it is brief, short, or of medium length, but highly variable in intensity, it is a shower. If it is steady and longer-lasting, it is rain.