The act of pouring or injecting something (liquid, funds, etc.). In medicine it refers to administering drugs or IV fluids; in economics it can mean injecting funds into a business or economy.
看護師が点滴で薬を患者の腕に注入した。
The nurse injected medication into the patient's arm via an IV.
政府は景気対策として企業に資金を注入した。
The government injected funds into companies as an economic stimulus.
新しいアイデアをプロジェクトに注入して、活気を取り戻した。
By injecting new ideas into the project, they restored its vitality.