Sure it is, the FDA defines spices as:

> SPICES - General Definition - Aromatic vegetable substances, in the whole, broken, or ground form, whose significant function in food is seasoning rather than nutrition. They are true to name and from them no portion of any volatile oil or other flavoring principle has been removed.

Ginger is also in the same family as cardamom and turmeric, and those would both be considered as spices as well.