WKT, or Well-Known Text, is a text-based format for geospatial data. It's very common in databases. Unlike formats like GeoJSON or KML, WKT can only store one feature at a time and has no way to store properties: formats that use WKT store properties in another way, like in other columns of a database table. In this sense, it's similar to the Polyline format, which can only store a single line - but WKT can store any kind of geometry.
Here's an example of a line encoded as WKT:
WKT is often stored and sometimes transferred as WKB, a binary version of the same format that has additional geometry types and is much more efficient to store and process.
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