**2. Dry eye disease (DED)**

"Dry eye disease (DED) is a multifactorial disease of the ocular surface characterized by a loss of homeostasis of the tear film, and accompanied by ocular symptoms, in which tear film instability and hyperosmolarity, ocular surface inflammation and damage, and neurosensory abnormalities play etiological roles." [6]. The description to understand this definition implies the term "multifactorial disease" recognizes DED as a significant and complex functional disorder that cannot be characterized by a single process, sign, or symptom. The term "ocular surface" encompasses structures including the tear film, the lacrimal and meibomian glands, the cornea, the conjunctiva, and the eyelids. "Homeostasis" describes a state of dynamic equilibrium in the body with respect to its various functions and the chemical composition of fluids and tissues. The key elements that contribute to the pathophysiological process, including tear film instability, hyperosmolarity, inflammation, and damage, recognized as etiological triggers of the vicious cycle, were considered important, along with neurosensory abnormalities, which have appeared more and more in recent literature, for inclusion in the definition [6].
