锁(suǒ) - lock; to lock in Mandarin

锁 (suǒ) is both the object and the action: a lock, and to lock. Chinese does not need a separate verb, so 一把锁 is a lock and 锁门 is to lock the door.

The measure word is 把, the same one used for keys, knives and umbrellas, anything with a handle or a grip. 一把锁 and 一把钥匙 travel together, and learners who reach for 个 will be understood but marked as beginners.

Note the resultative habit. Speakers rarely say a bare 锁门 when giving an instruction; they say 锁好 or 锁上, because the point is that the door ends up locked. The trap is that English "lock the door" hides that result, so the natural Chinese sentence carries one syllable more than the English one.