Primarily uncountable; rare plural form exists.
Translations
Carelessness can lead to serious errors.
Небрежность может привести к серьёзным ошибкам.
His carelessness cost the team the match.
Его невнимательность стоила команде победы.
The accident was a result of carelessness.
Авария стала результатом халатности.
Definitions
A lack of attention, thought, or effort; negligence. His carelessness behind the wheel led to the crash.
An act or instance of being inattentive or negligent. Several small carelessnesses added up to a costly mistake.