(n.) a single needle-like growth or projection, particularly of frost or ice
A spicula, or a spiculum, is a single needle-like prong of ice or frost.
In that context, the word spicula dates back to the early 1700s, but before then it was used much more generally in English to refer to any sharp, narrow, needle-like projection or point; the very earliest record of the word in English we know about refers to the crystalline spiculae of a raw sample of arsenic. In other contexts, the word has been used to refer to pine needles, the spines and pickles of quilled animals, the spike-like growths of certain fungi, and the slender, tapering point or shard of a fractured bone.
The word spicula itself is of course Latin. For obvious reasons, it literally means ‘little spike’.