What is Horner’s syndrome?

Horner’s syndrome: 

It is a syndrome due to the lesion in the cervical sympathetic pathway characterized by:

• Partial Ptosis
• Miosis (Pupillary constriction), reacts to direct and consensual light.
• Enophthalmos
• Anhydrosis (absence of sweating in the affected side of the face, whole upper limb and upper part of the trunk)

Related question: What are the causes of Horner's syndrome?

Additional:

What is the cervical sympathetic pathway?

It originates from the sympathetic nucleus in the hypothalamus and passes through the brain stem to the lateral horn of C5 and T1 segment of the spinal cord. From there, preganglionic fibres emerge and pass to sympathetic ganglia (usually superior cervical ganglia). Then the postganglionic fibres pass in the carotid sheath with the internal carotid artery, enter the skull along with it and in the cavernous sinus and join with the ophthalmic division of trigeminal nerve. Then it enters into orbit via short ciliary nerve and supplies the dilator pupillae, Muller’s muscle and sweat gland on the side of the face.