Crinum x. powellii 'Album'.
I think this is the first year this all-white crinum has bloomed in our garden. Last year, we had our first blooms from the "milk-and-wine" type, which are white with a red stripe down the center of each petal.
Speaking of the milk-and-wine crinum, this was supposed to be another of those, but when it bloomed this year (for the first time in our garden), it was clear that it's another type-- maybe 'Ellen Bosanquet'? (Though we have a small 'Ellen Bosanquet' elsewhere in the garden, it hasn't bloomed, yet, so I've only seen the flowers in photos.)
Whatever it is, I love the dark pink flowers!