What term is used to describe a split and brittle nail?

The term "onychorrhexis" specifically refers to the condition characterized by split and brittle nails. This term encompasses the splitting of the nail plate along its length, which often results in a rough texture and frayed edges. The condition can be caused by various factors, including overexposure to water, harsh chemicals, and a lack of moisture, which lead to the structural weakness of the nails.

In contrast, onychomycosis refers to a fungal infection of the nails, which affects the nail's appearance but is not directly related to brittleness or splitting. Leukonychia describes white spots or discolorations on the nails, often due to trauma, but does not imply brittleness. Trachyonychia is a condition marked by the occurrence of numerous tiny ridges or longitudinal streaks on the nail, which also does not align with the description of split and brittle nails.

Thus, onychorrhexis is the accurate term relating to the structural issues indicated in the question.