1An 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.
2Life; 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).
3Destiny 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.