acus

Acus is a Latin noun meaning "needle." In scientific terminology, acus functions as a root that appears in several derived words that describe needle-like shapes or projections. In anatomy and botany, terms such as acicular describe slender, needle-shaped structures; aculeus (a small needle or spur) and aculeate refer to small, pointed projections. The use of acus as a root helps describe microscopic or morphological features across diverse disciplines, including plant morphology, insect anatomy, and mineralogy.

In mineralogy and crystallography, acicular crystals are slender, needle-like formations that characterize certain minerals. The term describes habit rather than a specific mineral, and acicular crystals can occur in a variety of mineral species under appropriate growth conditions.

In contemporary English, acus by itself does not denote a standalone concept; it is most often encountered as part of compound terms derived from Latin. Its recognition and usage depend on the scientific context and the Latin etymology of the word. See also related forms such as acicular and aculeus for broader understanding of needle-like morphology.