A common directory on a file system which acts as a "scratch area" that can be used to hold files and directories for short periods of time.
When a file system is executing some process, it is often desirable to record information outside of registers/RAM and save them into memory. However, if these files are not intended to be permanently stored, a temporary "temp" directory may used to facilitate the storage/disposal of these files. For example, you might have a temp directory which stores logging data, which is regularly cleaned by the OS at discrete intervals.
temp may also refer to individual files, e.g. temp_data.csv which would be a temporary data file in the CSV format.