An age ending in zero, used especially when one turns that age on one's birthday. So, you're turning the big three-o tomorrow! How are you feeling about it?My mom's going to be the big six-o this summer, so we're planning a big family vacation to mark the occasion.See also: big
the big — -o
used to denote an age ending in a zero, especially as marked by a landmark birthday. informalSee also: big