Forming Adjacencies

[typesh]

Hey,

Trying to differentiate between Neighbors and Adjacency.
I understand that Neighbors will be two interfaces on a common network (such as a serial link). My question is more regarding Adjacency. Do two routers have to be directly connected to form an adjacency? Or, can routers form adjacencies with routers that aren't directly connected (but still in the same Area)?

Thanks!