String diameter
The diameter of the strings in the set, from thinnest to thickest.
Thicker strings, all other things being equal, provide more volume and richness to the sound, have more overtones, and generally sound richer and warmer (high frequencies are less pronounced in the sound). However, these strings require more tension, which can make them uncomfortable for beginners and inexperienced players who are unaccustomed to high force; and the duration of the sound is relatively low. Conversely, thinner strings are softer to the touch, require less effort, give better sustain, but are thinner and more prone to rattle. Detailed recommendations on the optimal string thickness for different cases can be found in special sources (including documentation for the instrument itself and reference materials from string manufacturers).
Note that the diameter of the strings is directly related to the tension (see below), so one of these parameters is often indicated in the characteristics.