Historically, defined an implicit data structure (and algorithms acting on one) as one "in which structural information is implicit in the way data are stored, rather than explicit in pointers." They are somewhat vague in the definition, defining it most strictly as a single array, with only the size retained (a single number of overhead), or more loosely as a data structure with constant overhead (). This latter definition is today more standard, and the still-looser notion of a data structure with non-constant but small overhead is today known as a succinct data structure, as defined by ; it was referred to as semi-implicit by .