SPST and SPDT are abbreviation commonly used to describe switches. SPST is an abbreviation for Single-Pole Single-Throw, and Pole refers to the number of circuits that one switch can control "simultaneously". Throw also represents the number of outputs that can be switched.
For example, SPDT is SP (Single pole) because one switch can control one circuit at a time, and DT (Double throw) because it can switch two outputs. (Table-1)
Table-1 Function name of Bus switches
The following documents also contain related information.