A single song
a long-established musical artist or band
. It is The One
that is typically thought of when the singer, musician or group that is famous
for performing and recording it is mentioned in any way. It differs from the single song a One Hit Wonder
makes famous because it stands out from a large body of well-known hits.
A signature song need not necessarily be written by the performer or musical act that is best known for it. No one can have more than one signature song, and it must be one that critics and fans alike generally agree on. You pretty much have to be very famous for a very long time before you are credited with such an accolade. Even then, it's something special.
For example, Elvis Presley, the Beatles and Bob Dylan all have many famous songs to their credit (a few of which may stand above the others) such that a group of ten fans would disagree upon what their signature song might be, so none of these legends have a signature song. But other giants like Tony Bennett ("I Left My Heart In San Francisco"), Led Zeppelin ("Stairway To Heaven") and Frank Sinatra ("My Way") have a single song that they are more famous for than all their others combined, and very few people dispute its distinction. This is a signature song.
Signature songs are pretty rare because they are a perfect combination of a number of important qualities a song must have in order to become timeless. The melody is memorable and powerful. The lyrics are inspiring, poignant, and convey ideas or emotions that every human being can identify with in a very personal way. The song's appeal is universal, and it is superior in style and distinction to any other that the artist has ever performed. When you hear it, for the first time or the thousandth time, it brings a smile to your face, and it lives forever in your heart. It's magic.
And yet, the perfect song alone is not enough. A signature song is a gift to its performer, given by fans and critics. This happens when the song is so fitting for the artist (because of personality, vocal delivery, musical arrangement or other esoteric qualities) that it is judged to be that artist's magnum opus. Timing and luck undoubtably play a role as well. Other musicians may perform or record the song, but it can never quite match the feeling that you get when you listen to the signature artist's version. A signature song is the perfect match between an artist and the one perfect song that expresses their style and mastery in the perfect way.