Euhedral and anhedral are terms used to describe opposite properties in the formation of crystals.
Euhedral (also known as idiomorphic or automorphic) crystals are those that are well-formed, with sharp, easily recognised faces.
The opposite is anhedral (also known as xenomorphic or allotriomorphic), which describes rock with a microstructure composed of mineral grains that have no well-formed crystal faces or cross-section shape in thin section.
An intermediate texture with some crystal face-formation is termed subhedral (also known as hypidiomorphic or hypautomorphic).
The flat faces (also called facets) are oriented in a specific way relative to the underlying atomic arrangement of the crystal: They are planes of relatively low Miller index.