A sidecar is a container that runs alongside another container as part of the same deployment unit. Just because two containers are in the same cluster or deployed around the same time doesn't make one a sidecar. There are two things that make a sidecar. First is that they share a network namespace, so they can reach each other over localhost rather than a network address. Second, they share a lifecycle. This means that they are created together, scaled together, and by default torn down together. Neither container has an existence independent of the other.