1. A the 1st letter of the Roman alphabet
  2. AIDS a serious (often fatal) disease of the immune system transmitted through blood products especially by sexual contact or contaminated needles
  3. ADHD condition characterized by behavioral and learning disorders
  4. A.D. in the Christian era
  5. AWOL one who is away or absent without leave
  6. 1000 the cardinal number that is the product of 10 and 100
  7. A.M. before noon
  8. ASAP as soon as possible
  9. Abel (Old Testament) Cain and Abel were the first children of Adam and Eve born after the Fall of Man; Abel was killed by Cain
  10. ATP a nucleotide derived from adenosine that occurs in muscle tissue; the major source of energy for cellular reactions
  11. ACTH a hormone produced by the anterior pituitary gland that stimulates the adrenal cortex
  12. 9/11 the day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs
  13. 24/7 uptime is 24 hours a day 7 days a week
  14. AMEX a stock exchange in New York
  15. 12th coming next after the eleventh and just before the thirteenth in position
  16. 20th coming next after the nineteenth in position
  17. AARP an association of people to promote the welfare of senior citizens
  18. 70th the ordinal number of seventy in counting order
  19. ARDS acute lung injury characterized by coughing and rales
  20. 30th coming next after the twenty-ninth in position