Victoria water lily, Amazon water lily (Victoria amazonica).
Victoria water lily is a rhizomatous, floating-leaf, tropical plant that is noted for the huge size of its yellow-green leaves (4-6 feet in diameter). Flowers bloom at night (open at dusk and close by noon), but each flower typically lasts only 2-3 days. Flowers open white but turn pinkish to rose-purple on the second to third days as they age. Victoria water lily is native to quiet shallow waters in tropical South America, including the backwaters of the Amazon River.
If you want more information about this flower, just search the plant's Latin (botanical) name on Google: