The interaction between a flower and a butterfly is called
Mutualism
Mutualism is a relationship between two organisms where both of them benefit from each other. This is also called symbiosis. Butterflies obtain nectar from flowers and in the process pick up pollen grains from the flowers. When they visit the next flower, some of the pollen grains get transferred, thus resulting in pollination. So, both the plants and the butterflies benefit from this interaction.