密码 : mìmǎ - password / PIN

密码 (mìmǎ) is "password" or "PIN" (thanks to Michael for this one), more literally a secret symbol or number. 密 (mì) is "secret", seen as part of 秘密 (mìmì) the word for "secret".

码 (mǎ) is "symbol" or "number".

If you're a Bank of America customer (at least in Boston) you'll see 密码 as the last two characters in the line of Chinese on the ATM screen when you log in:
請輸入您的個人密碼 (qǐng shūrù nín de gèrén mìmǎ), "Please enter your personal identification number" (The characters are traditional, like much of the printed material in Boston intended for Chinese speaking residents). 输入 (shūrù), is "input" or "enter". 输出 (shūchū) is "output".