What could a red raised itchy patch of skin on the back of the neck be?

Eczema vs dermatitis: This sounds like inflammation of the skin localized to one spot. Dermatitis is a fancy term for inflammation of the skin. Try treating with either a topical steroid such as Cortaid (hydrocortisone) or an anti-itch lotion such as sarna or sarna for sensitive skin. If this does not resolve it, see your doctor.

PSORIASIS,: Eczema or a fungal/bacterial dermatitis. It is important to have a doctor look at it, as the appearance and history of how it developed matters quite a bit in order to make a diagnosis and treat it for you.