Image
In plain terms
A packaged, immutable template for creating containers.
Going deeper
A container image bundles an app and its dependencies into a read-only template from which running containers are instantiated. It ensures consistent runs.
Under the hood
Built in layers (cached and shared); images are immutable, with containers as their running instances.
Examples
Docker images, Dockerfile builds, image registries, base images, tags.
Think of it like...
Like a cookie cutter from which identical containers are stamped out.