This is an excellent question.
Both are skin conditions and as Sarah explained so well here, the terms are considered to be interchangeable.
Here is a good way to remember the distinction:
Atopic dermatitis is a sub-class of eczema, but all eczema is not forms of atopic dermatitis.
Atopic dermatitis is the most common type of eczema. From a medical perspective, atopic dermatitis and atopic eczema are interchangeable terms. It is helpful to think of eczema as an umbrella term that is commonly used to refer to atopic dermatitis and the other forms of dermatitis and eczema underneath it (more information on them here: https://atopicdermatitis.net/?p=161). In the community, many people refer to atopic dermatitis as "atopic eczema" or just "eczema". Some may even refer to the condition as "AD" or "dermatitis" for short.