From the Latin form (Cassius) of the ancient Greek name Kassios(m), Kassia(f). Kassios was one of the many epithets of Zeus. Kassia was a byzantine poet.
Cassia (kassee-ah) can also be used to signify literally "Cassia", or the Cassia tree, from which cinnamon bark is obtained. It can thus be considered a plant or spice name. In this sense it is a direct translation of the Hebrew name Ketzia (which appears in the Bible as Keziah, one of Job's daughters).
This is one of my all-time favorite names. I really like the name Cassie, but to me, it does not seem formal enough for a full name, since it is usually a nickname. I'm not wild about Cassandra. So I love Cassia, because it features the name Cassi within it (and can still be used as a nickname) but on its own, it sounds simply beautiful.
I first heard this name was when I was about ten or eleven, and my class at school were doing a project on the European Union. A Polish interpreter called Kasia came to talk about the EU, and tried (in vain) to teach us some basic Polish expressions (the only one of which I can remember is czeϾ, which means hello). Since then, I've always thought that Kasia is a cool and very beautiful name. Now I prefer this Cassia spelling, because it makes me think of cinnamon, and it's one letter away from _Cassis_, an awesome song by the GazettE.
Cassia is a very, very pretty and elegant name, when pronounced KAH-sha. When pronounced KASS-ya, it loses a lot of its elegance and sounds like the pretentious Alicia. It also reminds me of the spice Cassia, which is similar to cinnamon.