Is atopy different thing from atopic dermatitis?

They're related: Atopy is the medical term lay folks equate with "allergy". It's a general term. Atopic dermatitis is a particular form of atopy. Specifically it's an allergic response within the dermis, which is the connective tissue under the skin. Many know it as eczema. People can have chronic issues exacerbated by flare ups. Pts can often be treated by their pcp. Derm referral may be needed for severe cases.

Not really: Atopy is a general term of allergy that can be applied to any allergic manifestation such as asthma (atopic asthma, i.e.) whereas atopic dermatitis further narrows the notion of atopy to skin only entity.