1. Needle-like; referring to a stout bristle or an extended and pointed structure: The slender leaves of pine trees, for example, are quite acicular.

A hummingbird has an acicular bill being long and narrow.
2. In metallurgy, when referring to cast iron or an alloy possessing a microstructure of pointed components: Some minerals showing acicular forms include mesolite, malachite, and gypsum.

In hydrology, freshwater ice made up of many long crystals; fibrous ice: satin ice: Such acicular ice contains air bubbles and has many long, needle-like crystals and hollow tubes in a layered arrangement.