行為だ

is an act

→ 行為
phrase Uncommon #14,023
is an act 行為 refers to an act or deed, often one being judged or characterized.
JA: そんなことをするのは危険な行為だ。
EN: Doing something like that is a dangerous act.

それは愚かな行為だ。

That is a foolish act.

降りるのは自殺行為だ

Getting off would be suicide.

— プラットフォーム (2019)

それはまさに侵略行為だ。

That is nothing short of an act of aggression.

自殺は絶望的な行為だ。

Suicide is an act of despair.

自殺行為だよ

That’s suicidal.

— どん底作家の人生に幸あれ! (2019)

捕虜を虐殺したのは残忍な行為だ。

Massacring prisoners of war was a brutal act.