What does the phrase "a separate peace" mean?

A Separate Peace refers to the brief period of peace that the Devon boys experience at school while WWII was being fought. The knowledge that they could end up in the war themselves hovered at the edges of their time at the school.
The phrase, or title, also refers to the relationship between Gene and Finny. Their friendship is full of tension, in part because of Gene’s envy at the ease which he perceives Finny moves through life. When Gene shakes the branch that causes Finny to fall and break his leg, he wrestles with his own intentions and the outcome of that day. Later, when Finny dies after a surgery related to the injury he received when Gene shook the branch, Gene is overcome by guilt. Eventually he is able to come to peace, realizing that he has become himself by adopting Finny's best qualities.