What does "PST" stand for in Outlook?

In Microsoft Outlook, “PST” is the file extension for the primary data file format used by the software, the Outlook data file. Outlook data files, which have a “.pst” extension, are used to store email messages and other Outlook-associated data.

Outlook data files are typically used to archive and backup the data contained in Outlook email folders. Such archive files reside on a user’s computer and thus have no file size limits apart from those arising from the size of the user’s hard drive. An Outlook data file can also be used to offload data from an online account. The data can then be accessed seamlessly on the user’s computer.