a title track is the song a release (typically album) is named for. this isn't limited to the title of the song, as many title tracks only have the name of an album in the lyrics. some examples:

• Zeitgeist is the title track of TULPA (from the lyrics, "I'm a tulpa")
• I’ll Wait is the title track of I’ll Wait
• Eclipse is the title track of The Dark Side of the Moon (from the lyrics, "I'll see you on the dark side of the moon")

...and many more in the 'title track' tag