is an expectation of zero. What you ultimately stand to win (or lose) in such a scenario is zero. You break even.
One example is a fair coin toss, betting for heads or tails. If you bet an x amount for heads or tails, and receive 1:1 payout, you will break even in the long run, because you are expected to win and lose half the time.
Another example of a zero sum scenario is taking the insurance bet when 1/3 of the remaining cards in the shoe or pitch are 10�s. We have a 2:1 payout here. So, we are expected to win twice our bet 1/3rd of the time and lose our bet the remaining 2/3rds of the time, yielding a zero sum, or expectation.
Choosing to double or not on a soft 15 vs 4 off the top of a deck is also close to zero sum.
Another good example, which a gentleman here posted a while back, is the change machine. You put, for example, a dollar into a change machine, and then you are given back 4 quarters. This is a zero sum scenario, and also specifically a zero variance scenario.