General Antonyms:
* Retract: This is the most direct opposite, meaning to draw back or in.
* Recede: This suggests moving back or away from a point of view.
* Withdraw: This implies pulling back or removing oneself from a situation.
Specific Antonyms:
* Indent: If "protrude" refers to sticking out, "indent" means to make a hollow or sunken area.
* Recess: If "protrude" describes a part extending outward, "recess" means a space that is set back or indented.
* Conceal: If "protrude" means to be visible, "conceal" means to hide or make something invisible.
Examples:
* "The nail protruded from the board" could be replaced with "The nail was recessed into the board" or "The nail was hidden within the board."
* "The mountain range protruded from the plain" could be replaced with "The mountain range receded into the plain."
The best antonym for "protrude" will depend on the specific meaning of the word in your sentence.