1. A person who is the replacement for a significant other, relative, or friend that isn't around anymore.
2. Derived from a quote by Claire from Elizabethtown.
"You and I have a special talent," Claire says to Drew. "And I saw it immediately. We're the substitute people. I've been the substitute person my whole life. I'm not an Ellen a co-worker Drew was into. I never wanted to be an Ellen. And I'm not a Cindy either…I like being alone too much. I mean, I'm with a guy who is married to his academic career. I rarely see him and I'm the substitute person there. I like it that way. It's a lot less pressure."
"Since Anne just broke up with Joe, Emily is Joe's substitute person."