In 3D graphics and games, batching is a very general term used to describe the process of grouping a large number of wayward pieces of data together and processing them as a single, large block of data. This situation is ideal for CPUs, and particularly GPUs, which can handle the simultaneous processing of multiple tasks with their multiple cores. Having a single core switching back and forth between different locations in memory takes time, so the less this needs to be done, the better.
In some cases, the act of batching refers to large sets of meshes, vertices, edges, UV coordinates, and other different data types that are used to represent a 3D object; however, the term could just as easily refer to the act of batching audio files, sprites, texture files, and other large datasets.
So, just to clear up any confusion, when the topic of batching is mentioned...