TIA's Glossary of Telecommunication Terms
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core area
The part of the cross-sectional area of an optical fiber within which the refractive index is everywhere greater than that of the innermost homogeneous cladding, by a specified fraction of the difference between the maximum refractive index of the core and the refractive index of the innermost cladding. Note 1: Artifacts of the manufacturing process, such as refractive index dip, are ignored in computing the points (refractive indices) of demarcation. Note 2: The core area is the cross-sectional area within which the refractive index is given by 
where n 1 is the maximum refractive index of the core, n 2 is the refractive index of the homogeneous cladding adjacent to the core, n 3 is the defining refractive index, and m is a fraction, usually not greater than 0.05. [After 2196]