1. An album, CD, or other recording by an artist that is considered, or recommended by the artist's fans, to be the best introductory recording for one who is not completely familiar with the artist's work, or for one who has not listened to a complete album, CD, or other recording by an artist.
2. A release from an artist which has the greatest likelihood of being enjoyed and appreciated by one who has not listened to any of the artist's entire recordings, or by one who has only heard a few songs by the artist.
3. A recommendation by an artist's fan or listener that will, in the fan or listener's opinion, be the most appropriate recording to listen to as an introduction to the artist, or that is recommended to be the first recording by the artist that a new listener acquires and listens to.
Yes's "Fragile" from 1971 is a good starter album because you can hear the influence and virtuosity that each of the band members contributed to the songs. It is their classic, and quintessential sound that improves on their past recordings and then permeates their next several releases.