Communication
The conveying of meaning and intent through visual signs, symbols, and systems.
In practice
Communication in design means getting a specific message and intent across through visual means: words, images, signs, color and layout working together. The test is whether the intended audience understands what was meant, not whether the designer is pleased. A hospital sign communicates well if a stressed visitor can find the emergency department at a glance, whatever its style.
Often confused with Interpretation
Communication is the sender's intended meaning being conveyed; interpretation is how each viewer personally understands it, which may differ.
Try this
Show a sign or poster you made to someone for five seconds, then ask what it told them to do.