Abbreviation of 電子計算機 (electronic computer). Used mainly in compound terms like 電算機 or 電算室, and in older or technical contexts. In modern everyday language, コンピューター is far more common.
See also: 電子計算機
In this company's computer room, large computers are lined up.
電算機の歴史について調べている。
I'm researching the history of electronic computers.
A shortening of 電子計算機, “electronic computer,” or the associated form 電子計算, using 電 from 電子 and 算 from 計算.