…’An agent is just something that acts (agent comes from the Latin agree, to do). Of course, all computer programs do something, but computer agents are expected to do more: operate autonomously, perceive their environment, persist over a prolonged period, adapt to change, and create and pursue goals’… 
agent and environments
…’ An agent is anything that can be viewed as perceiving its environment through sensors and acting upon that environment through actuators’…
…’ Term percept is used as a reference to the agent’s perceptual inputs at any given instant. An agent’s percept sequence is the complete history of everything the agent has ever perceived. […] By specifying the agent’s choice of action for every possible percept sequence, we have said more less everything there is to say about the agent. Mathematically speaking, we say that an agent’s behaviour is described by the agent function that maps any given percept sequence to an action’… 
1 Russell, S., Norvig, P., 2016. Artificial Intelligence: A Modern Approach, Third Edition. ed. Pearson Education Limited .