Robert Altman, one of the most adventurous and influential American directors of the late 20th century, a filmmaker whose iconoclastic career spanned more than half a century but whose stamp was felt most forcefully in one decade, the 1970s, died Monday in Los Angeles. He was 81. His death, at a hospital, was confirmed today by a friend, the singer Annie Ross. The cause was not announced. Mr. Altman had a heart transplant in the mid-1990s, a fact he publicly revealed for the first time last March while accepting an honorary Oscar at the Academy Awards ceremony.