For a tree trunk:
* tronco (pronounced "tron-koh") is the most common and general term.
For a car trunk:
* maletero (pronounced "mah-leh-tay-roh") is the most common term in Spain.
* cajuela (pronounced "kah-hoo-eh-lah") is the most common term in Latin America.
For a chest or suitcase:
* baúl (pronounced "bowl") is the most common term.
For the trunk of an elephant:
* trompa (pronounced "trom-pah") is the word for the elephant's trunk.
So, to be precise, you need to specify what type of "trunk" you are referring to.