Tacit communication: How's it going?
It happens like this: you are walking from your desk to the conference room or the kitchen and you see Janet from Accounting walking toward you from the opposite direction. As you pass you both nod and smile a little smile. She says "How are you doing?" and you say "Hi, Janet."
You acknowledged each other. You made each other feel good, known, loved, recognized. But neither of you expected the other to stop and engage in a real conversation. You both knew the "rules of non-engagement" and kept on walking. This can happen dozens of times throughout the day with various people.
This is tacit communication. The actions - the nod, the smile - provide more meaning than the actual words exchanged.