必要 (bì yào) is an adjective meaning 'necessary, essential' — it describes something needed for a purpose. Unlike 必需 (bì xū), which is a verb meaning 'to need/require' (e.g., 必需品 'necessities'), 必要 modifies nouns directly (必要的条件 'necessary condition'). For a stronger, more formal sense of 'indispensable', use 必不可少 or 不可或缺.