The Collaborative International Dictionary of English v.0.48:

flexitime \flex"i*time`\, flextime \flex"time`\ flex time \flex"
time`\, n. [short for flexible time.]
   A system of scheduling working hours in places of employment,
   which allows employees to arrive at and leave work at times
   of their own choice, providing that they work the required
   number of hours and usually requiring that they be present
   during certain hours, called core time.

   Note: Certain workers prefer to arrive early and leave early,
         and others prefer to arrive late and leave late. Flex
         time is a way to accommodate such preferences,
         presumably improving employee morale, and also helps
         reduce traffic congestion during commmuting times.
         [PJC]