A Styled Layer Descriptor (SLD) is an XML schema specified by the Open Geospatial Consortium (OGC) for describing the appearance of map layers.
Overview
An SLD is capable of describing the rendering of vector and raster data. A typical use of SLDs is to instruct a Web Map Service (WMS) how to render a specific layer.
Other references
Styled Layer Descriptor (SLD) - Wikipedia
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