the form (or mood) of a verb that expresses wishes, possibility or uncertainty; a verb in this form 虚拟式;虚拟语气
The verb is in the subjunctive.
这个动词是虚拟语气。
In ‘I wish I were taller’, ‘were’ is a subjunctive.
在 I wish I were taller 中,were 是虚拟式。