Two tasks completed, achieved, or taken care of at the same time or with a singular series of actions; two problems solved with one action or solution. Typically used with the verbs "kill" or "hit." I might as well kill two birds with one stone and drop off my tax return while I'm at the mall for the computer part I need. Bike-to-work plans are a great way of hitting two birds with one stone: getting more exercise while cutting down on the cost of your daily commute.