Different plants that are planted together for the benefit of each other. Whether it be color or roots deeper to bring up the nutrients for the secondary plant. Ground covers are great companion plants.
The act of planting two different plants within close proximity of each other with the belief that traits from each plant will benefit the other.
Some plants inhibit the growth of others nearby by releasing substances into the soil (called allelopathy), others actually assist others by fixing nitrogen or by repelling insects that attack them (oregano repels the cabbage butterfly for example). Gardeners can profit by being aware of these relationships. Process is called companion planting.