وهو ما وضع لشيء معين مع جميع مشخصاته
That is, a word assigned to a particular thing considered with its individuating characteristics.
Designation (وضع) is assigning a linguistic expression to a referent. Individuating features (مشخصات) distinguish one individual from others. The definition does not mean that every property must be verbally listed whenever the name is used; it means the naming act concerns a particular referent rather than an unrestricted class.
«رَجُلٌ» can apply to many men, while «زَيْدٌ» functions as the name of a particular Zayd. If several people share that name, context may be needed to select the intended one; this does not turn the word into an ordinary class noun.