A technical name for a slender needle-like body or part: The aciculae are the spines or prickles which some animals and plants are furnished with, or the needle-like crystals of certain minerals.

1. A needlelike spine or bristle: An aciculum is present in certain types of worms or flagellates.
2. A projecting cluster of bristles or setae: An aciculum is found in certain annelid worms, or is a fingerlike or handlike bristle seen in certain flagellate protozoa.
3. In zoology, one of the slender, sharp stylets embedded in the parapodia of certain annelid worms: Small needlelike appendages, or the aciculua, are lodged in the fleshy extremities of polychaetes.
4. A technical name for a slender needlelike body: The spines or prickles of some animals and plants are termed acicula, but also the needlelike crystals of certain minerals are called the acicula.