Tension Headache: This is one of the most common types of headaches and can be unilateral. Given the age and the lack of specific details suggesting other conditions, tension headache is a plausible initial consideration due to its high prevalence.

Migraine: Although migraines are more common in younger populations, they can occur at any age. A new onset of migraine in an older adult, especially if accompanied by other symptoms like aura, nausea, or sensitivity to light and sound, should be considered.
Temporal Arteritis (Giant Cell Arteritis): This condition is more common in older adults and can cause unilateral head pain, typically in the temporal region. It's a significant consideration due to its potential to cause blindness if not treated promptly.
Dental or Sinus Issues: Problems such as toothaches or sinusitis can refer pain to the head and can be unilateral, depending on the location of the issue.

Subarachnoid Hemorrhage: Although less common, a sudden, severe headache (often described as "the worst headache of my life") could indicate a subarachnoid hemorrhage, which is a medical emergency.
Stroke or Transient Ischemic Attack (TIA): Headache can be a symptom of a stroke or TIA, especially if accompanied by other neurological deficits such as weakness, numbness, or difficulty with speech.
Temporal Arteritis (Giant Cell Arteritis): Also listed under other likely diagnoses, it's crucial enough to be mentioned here as well due to its potential for serious complications, including blindness.

Trigeminal Neuralgia: This condition causes sudden, severe, shock-like pain around the eyes, lips, or nose mainly because of abnormal blood vessels pressing on the trigeminal nerve in the head. It's rare and typically affects one side of the face.
Occipital Neuralgia: Involves inflammation or irritation of the occipital nerves, which can cause severe pain on one side of the head, typically starting from the base of the skull and moving up.
Hemicrania Continua: A rare form of headache that is continuous and strictly unilateral, often accompanied by autonomic symptoms like nasal congestion or eye watering on the same side as the headache.