An order, command, or decree (can be used to mean an instruction or order).
長官の命で調査が始まった。
An investigation began by the commanding officer's order.
上官の命に従わなければならない。
You must obey your superior's orders.
Life; the state of being alive (often written/read as いのち in everyday speech).
医者は患者の命を救った。
The doctor saved the patient's life.
命あっての物種だ。
As long as you're alive, there is hope (life comes first).
Destiny or fate; the predetermined course of someone's life.
彼女はそれが自分の命だと受け止めた。
She accepted that as her fate.
運命は時に残酷だと感じる。
Sometimes fate feels cruel.