An atomic operation.

Of or relating to atoms; composed of atoms; monatomic.

"A stream of atomic hydrogen is emitted."

Employing or relating to nuclear energy or processes.

"We built a small atomic bomb in the garage."

Infinitesimally small.

"The hairs on a bedbug are almost atomic."

Unable to be split or made any smaller.

"A bit is an atomic item of data."

Of an operation: guaranteed to complete either fully or not at all while waiting in a pause, and running synchronously when called by multiple asynchronous threads.

"In order to avoid race conditions, this operation has to be atomic."

(of a commit in a VCS) Containing a single change, as opposed to involving numerous unrelated changes.