birth·day
 /ˈbɚθˌdeı/  noun  ,  pl   -days [  count  ]
  1 : the day when someone was born or the anniversary of that day
   Her birthday is September 30th.
   Today is his 21st birthday.
   Happy Birthday!
  — often used before another noun
   a birthday party/gift
  2 : the day when something began
   The company just celebrated its 50th birthday.
  birthday suit
    informal + humorous  
   ◇ If you are wearing your birthday suit, you are naked.
    We were standing there in nothing but our birthday suits.