Lorna Bucket was a young woman who fought on the Doctor's side in the Battle of Demons Run ("A Good Man Goes to War") & was killed by the Headless Monks. As a child, she had met the Eleventh Doctor in the Gamma Forests, as related in "Lorna's Escape" (short story). When she was dying, he pretended to remember her, even though he had no memory of her because their meeting was still in his future.
Lorna indirectly & unintentionally gave Melody Pond the name by which she's more often known: River Song. (See How is Melody Pond also River Song#Names for the explanation of this.)