The noun 司机 (sījī) means 'driver,' referring to a person who operates a vehicle such as a car, bus, or truck. It is a common everyday word used in both formal and informal contexts. It can be modified by the type of vehicle, such as 出租车司机 (taxi driver).
司机师傅,请在前面路口停一下。
Sījī shīfu, qǐng zài qiánmiàn lùkǒu tíng yíxià.
Driver, please stop at the intersection ahead.
她考了驾照以后当了一名出租车司机。
Tā kǎo le jiàzhào yǐhòu dāng le yì míng chūzūchē sījī.
After she got her driver's license, she became a taxi driver.
那个公交车司机人特别好。
Nàge gōngjiāochē sījī rén tèbié hǎo.
That bus driver is a really nice person.