"The Constant" is an orchestral piece on the Season 4 soundtrack. It plays at the end of the episode "The Constant".
Desmond dials Penny. In 1996, he walks outside her apartment and sees her close her curtains. In 2004, she picks up the phone. In 1996, Desmond smiles, and back in 2004, he starts crying with joy.
We see Penny in her London apartment in front of a Christmas tree. Desmond tells her he's on a boat near an island, and Penny tells him she's been searching for him, and Charlie's call convinced her he's still alive. They tell each other "I love you" before the phone goes dead.
Back on the Island, Daniel finds a message in his notebook saying that if anything goes wrong, Desmond will be his constant.