Read this interesting article with more details.

Simple explanation :

An atomic transaction is an indivisible and irreducible series of database operations such that either all occur, or nothing occurs.[1] A guarantee of atomicity prevents updates to the database occurring only partially, which can cause greater problems than rejecting the whole series outright.

In cryptocurrency this translates to a trustless exchange. Komodo's Barterdex is an atomic swap protocol that is blockchain agnostic and allows traders to swap tokens atomically across blockchains while remaining in control of their private keys.

[1] Not to be confused with a Lightning Network (LN) off chain atomic swap which does not guarantee the above.