1. A the 1st letter of the Roman alphabet
  2. Aborigine a dark-skinned member of a race of people living in Australia when Europeans arrived
  3. A horizon the top layer of a soil profile; usually contains humus
  4. 1000000000 the number that is represented as a one followed by 9 zeros
  5. Abraham the first of the Old Testament patriarchs and the father of Isaac; according to Genesis, God promised to give Abraham's family (the Hebrews) the land of Canaan (the Promised Land); God tested Abraham by asking him to sacrifice his son
  6. 1st class mail that includes letters and postcards and packages sealed against inspection
  7. A-horizon the top layer of a soil profile; usually contains humus
  8. 440 yards a unit of length equal to a quarter of 1 mile
  9. 880 yards a unit of length equal to half of 1 mile
  10. A battery the battery used to heat the filaments of a vacuum tube
  11. Abkhazian of or relating to Abkazia or its people or their language
  12. Aaron Burr United States politician who served as vice president under Jefferson; he mortally wounded his political rival Alexander Hamilton in a duel and fled south (1756-1836)
  13. Aberdeen a city in northeastern Scotland on the North Sea
  14. ABO group a classification system for the antigens of human blood
  15. 401-k plan a retirement savings plan that is funded by employee contributions and (often) matching contributions from the employer; contributions are made from your salary before taxes and the funds grow tax-free until they are withdrawn, at which point they can be converted into an IRA; funds can be transferred if you change employers and you can (to some extent) manage the investments yourself
  16. Abkhasian a Circassian language spoken by the Abkhaz
  17. ABO system a classification system for the antigens of human blood
  18. Abkhazia an autonomous province of Georgia on the Black Sea
  19. Aberdonian of or relating to Aberdeen
  20. AND gate a circuit in a computer that fires only when all of its inputs fire