Captain America: The Winter Soldier Audio Commentary:

“I think people miss the concept. When you call the movie ‘Captain America: The Winter Soldier’, the notion of the Winter Soldier is from a quote from Paine about Summer Soldiers and Winter Soldiers. Winter Soldiers are the real soldiers 'cause they are the soldiers who would fight through the winter. In a way, by Paine’s definition they are both Winter Soldiers, which is the great thing about the title.
People have said, "He is the titular character, why isn’t he more in the movie?” When really, Cap is the titular character. Cap is the Winter Soldier. The whole film is about a character who is the greatest of soldiers. He will fight through the darkest times and the coldest winter to see his principles through. That’s really what the movie is about.“