Does eczema usually have a defined boarder?

Not typically: Usually eczema is somewhat poorly defined as far as it's margins. Well demarcated lesions are more typical for ringworm (fungal) or some types of ichthyosis disorders.

Not usually: Some forms such as "nummular" eczema have well defined borders, but most often the condition consists of ill defined plaques of papules, scale, and excoriations.