相互作用 is a formal term for 'interaction' or 'to interact', emphasizing mutual or reciprocal action. It is commonly used in scientific, technical, or abstract contexts (e.g., interactions between particles, factors, or systems). As a verb phrase, it directly serves as the predicate (因素相互作用 'factors interact'), and it can also function as a noun (相互作用关系 'interaction relationship'). Note that 互动 (hùdòng) is more common for social or everyday human interaction, while 相互作用 leans toward physical, theoretical, or causal interactions.