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An act or action, a deed
El acto de donar sangre es muy noble.
The act of donating blood is very noble.
A formal or official ceremony or event
La ceremonia fue un acto solemne.
The ceremony was a solemn act.
In law, an act or statute
El acto legislativo fue aprobado por el congreso.
The legislative act was approved by congress.
Often used in legal and formal contexts.
From Latin 'actus', past participle of 'agere', meaning 'to do'.
Commonly used in both formal and informal contexts.