It had been dark in the room since the door was locked from the outside.

the 是定冠词，用于特指某个或某些特定的事物或人。 outside 是名词，意思是"外部、外面"，指物体或空间的表面或外侧部分。 the outside 在这里特指"门的外侧"，即门朝向外面的那一面。