In all the books I've read, Terra is Earth. In The Flight of the Eisenstein they mention the other planets of the solar system when they're bringing Garro and his men back to Luna (the Earth's moon) for holding and interrogation. Also, I forget which book it was, but there was mention made of the ancient Egyptian culture of terra (granted it wasn't spelled exactly the same way).
"There is a cancer eating at the Imperium. With each decade it advances deeper, leaving drained, dead worlds in its wake. This horror, this abomination, has though and purpose which functions on an unimaginable, galactic scale and all we can do is try to stop the swarms of bio-engineered monsters it unleashes upon us by instinct. We have given the horror a name to salve our fears; we call it the Tyranid race, but if it is aware of us at all, it must know us only as Prey."