This character is a Other Symbol and is commonly used, that is, in no specific script.
The glyph is not a composition. It has no designated width in East Asian texts. In bidirectional text it acts as Other Neutral. When changing direction it is not mirrored. The word that U+25B0 forms with similar adjacent characters prevents a line break inside it.
The CLDR project calls this character “filled parallelogram” for use in screen reading software. It assigns these additional labels, e.g. for search in emoji pickers: filled, parallelogram.
The Wikipedia has the following information about this codepoint:
In Euclidean geometry, a parallelogram is a simple (non-self-intersecting) quadrilateral with two pairs of parallel sides. The opposite or facing sides of a parallelogram are of equal length and the opposite angles of a parallelogram are of equal measure. The congruence of opposite sides and opposite angles is a direct consequence of the Euclidean parallel postulate and neither condition can be proven without appealing to the Euclidean parallel postulate or one of its equivalent formulations.
By comparison, a quadrilateral with at least one pair of parallel sides is a trapezoid in American English or a trapezium in British English.
The three-dimensional counterpart of a parallelogram is a parallelepiped.
The word comes from the Greek παραλληλό-γραμμον, parallēló-grammon, which means a shape "of parallel lines".