This is the 1017th most frequent Russian word.
действие
"Action" or "act."
The word 'действие' refers to an action or deed in this context.
Это действие было необходимо для успеха.
This action was necessary for success.
Here, 'действие' stands for an act or part of a theatrical performance.
Театральное действие началось поздно.
The theatrical act started late.
'действие' in this sentence implies a process or reaction in a scientific context.