Pitch is the distance between screw threads and is commonly used with inch sized products and specified as threads per inch.
Lead is the linear travel the nut makes per one screw revolution and is how ball screws are typically specified.
The pitch and lead are equal with single start screws. For multiple start screws the lead is the pitch multiplied by the number of starts.
VIDEO: Identifying your ball screw assembly
The video below illustrates some ball screw basic terms, including lead and pitch.