وجه الشبه
The shared aspect (wajh al-shabah) is the quality or meaning in which the two compared terms are related.
In “Zayd is brave like a lion,” it is courage. In “the child's face is bright like the moon,” it may be brightness or beauty, as determined by the context.
The two terms do not need to resemble one another in every respect. Simile generally selects one feature among many possible qualities. The speaker may intend a particular dimension even where several are potentially shared.
The shared aspect can be understood without being explicitly stated; omission from the wording does not entail absence from the comparison.