What is the likely condition if an elderly woman has a triangular thickening of the conjunctiva encroaching on the cornea?

The condition described—triangular thickening of the conjunctiva encroaching on the cornea—indicates the presence of a pterygium. A pterygium is a growth of fleshy tissue on the conjunctiva that can extend onto the cornea and is often associated with chronic exposure to sun, wind, or dust. This growth is usually triangular in shape and can sometimes lead to vision issues if it grows significantly.

In contrast, corneal arcus refers to a gray or white arc around the cornea and does not invade the cornea; it is typically a sign of aging or hyperlipidemia rather than a growth. A pinguecula is also a conjunctival growth but is typically yellowish and does not invade the cornea or have a triangular shape. It is more common in older adults but does not affect vision significantly. A chalazion is a small lump in the eyelid due to blocked oil glands and is distinctly different in appearance and location, as it does not involve the conjunctiva or cornea in this manner.

Thus, a pterygium fits the description provided and is the condition that most closely matches the symptoms presented.