The definition includes items which should not be included.
This fallacy is made when the definition allows many more items than those intended. This fallacy can be committed at the same time as a too narrow fallacy, if the definition also does not include items that should be included.
Of course, agreeing on a definition is sometimes difficult, especially when the definition is intensional. In general semantics terms, we speak of (over/under)-definition: over-defined by intension (belief in words) and under-defined by extension (lacking details in fact).
An apple is something which is red and round.
The planet Mars is red and round. So it is included in the definition. But obviously it is not an apple. Since Golden Delicious apples are apples but not red (they are yellow), they are not included in the definition, however, they should be. So this definition is also too narrow.
A figure is square if and only if it has four sides of equal length.
Not only squares have four sides of equal length; trapezoids do as well.
Identify the term being defined. Identify the conditions in the definition. Find an item which meets the condition but is obviously not an instance of the term. Using extensional definitions is useful.