This is a term which has persisted from the original NaNo Novel version of Destiny of Dreams . People who began their evolution on the earth and are therefor basically genetically linked to us are called Terrans.
Also in the NaNo Novel version, there were 16 "Terran" planets, which meant those planets were fully suited for human life. HomeWorld was Terran 3 and Paragangia was Terran 13. the only vestige of this term remaining in the current version is the name Terran 8. Ironically the only place still named Terran is the place NON-Terrans came from