What term describes a needle-shaped structure?

The term that best describes a needle-shaped structure is acicular. This word specifically denotes a shape that is slender and pointed, much like a needle. Acicular structures are commonly found in various biological and geological contexts, such as in certain crystals or plant fibers.

Linear refers to something shaped like a line, which does not necessarily convey the pointed quality of a needle. Belonoid, while related to shapes that might resemble a needle, is not widely recognized in standard terminology for medical or anatomical descriptions. Cylindrical describes a tube-like shape, which is not applicable to something that is needle-shaped.

In summary, acicular provides the precise definition needed for a needle-shaped structure, highlighting the significant characteristics that distinguish this shape from others.