From The New Yorker's 7/21 story "Making It: How Chicago shaped Obama":
"But his life in Chicago from 1991 until his victorious Senate campaign is a lacuna in his autobiography."
Lacuna -- [from Latin lacuna -- pool, pit, or gap; also the root of lagoon, from the French lagune and Italian laguna] pl. lacunae
1: a blank space or a missing part
2: a small cavity, pit or discontinuity in an anatomical structure