What is a 'second wave' of an illness?

Second wave: A phenomenon of infections that can develop during a pandemic. The disease infects one group of people first. Infections appear to decrease. And then, infections increase in a different part of the population, resulting in a second wave of infections.

If and when infection rates begin to climb again, that will indicate the next or "second wave." The longer the pandemic goes on, the more waves are likely to occur.