In simple terms, communication can be defined as actions whereby 'actors' impart information to one another.
Communication does not have to involve language and does not have to be vocalized. Deaf people often communicate through gestures. Baby's communicate basic wants through crying.
Communication does not have to be intended. When we become emotionally aroused we often unintentionally communicate our intense feelings to others through subtle shifts in body language, physiological changes and even pheromones.
We used the word actors in our definition because animals can communicate with each other and with us. A cat can demand dinner just as forcefully as any human.