a machine for performing calculations automatically
"The scientists relied on the supercomputer to perform millions of complex calculations automatically overnight."
an expert at calculation (or at operating calculating machines)
"The veteran accountant was known as a computer because she could crunch payroll numbers faster than anyone else in the office."
A person employed to perform computations; one who computes.
"During the early days of computing, they hired a computer to manually calculate complex payroll figures for the factory."
In plain English: A computer is an electronic device that processes data and runs programs to perform tasks for you.
"I used my computer to finish my homework online."
Usage: In modern usage, "computer" refers to an electronic machine that processes data and performs calculations, not a human being. Although the term historically described people who performed manual math, it now almost exclusively denotes digital devices like laptops or smartphones.
The word computer comes from the verb compute with the suffix -er added to form an agent noun. While it originally meant a person who performs calculations, this second meaning of a machine appeared around 1897 in the magazine Engineering.