The number of bases that are read at one time i.e the number of letters that will appear in each read. This differs between technologies so optimum fragment length varies.
Use in clinical context
Different sequencing technologies can process different read lengths. Short reads require the genome to be fragmented and then reconstructed, which has limitations but can be less error prone than long read sequencing. However, long read sequencing can help decipher parts of the genome such as repetitive sections that are not easily reconstructed using short reads. The technology used and the read length produced can depend on the clinical question.