Digital computer and Mainframe computer

Digital computer noun - A computer that represents information by numerical (binary) digits.

Mainframe computer and digital computer are semantically related. In some cases you can replace phrase "Mainframe computer" with "Digital computer", this phrases are similar.

Mainframe computer noun - A large digital computer serving 100-400 users and occupying a special air-conditioned room.

Digital computer and mainframe computer are semantically related. You can use "Digital computer" instead a noun phrase "Mainframe computer".