Octocrylene

Octocrylene is a chemical sunscreen ingredient that, once absorbed into the skin, captures harmful UV rays before they can cause damage to the underlying skin cells.  Octocrylene mostly offers protection against the shorter UVB rays (those responsible for burning) and not as much against the longer UVA rays (those responsible for signs of aging).

Octocrylene is quite photo-stable and doesn't break down or change its molecular structure in the presence of UV light, as some other chemical sunscreen ingredients do.  Octocrylene can also help improve the water-resistance of a sunscreen.

Octocrylene doesn't pose any substantial side effects to most people, though some studies have shown that children are more prone to having an actual allergy to the ingredient.  It's therefore suggested that children and anyone with sensitive skin or issues such as rosacea or eczema avoid it, as does anyone who's breast-feeding.
If you're unsure about it, pass on this and other chemical sunscreens, and stick with mineral options that include zinc oxide and titanium dioxide, two sunscreen ingredients the FDA has deemed "generally recognized as safe and effective."