Conversational Agent
A conversational agent, or conversational system, or dialog system, is a computer system (hardware+software platform) intended to converse with a human in a natural language.
Let's see what wikipedia states for conversational agent[1]:
"A dialogue system, or conversational agent (CA), is a computer system intended to converse with a human. Dialogue systems employed one or more of text, speech, graphics, haptics, gestures, and other modes for communication on both the input and output channel."
In common practice, conversational agent in an umbrella term generally used as synonym of chatbot, but technically a chatbot is a specific kind of conversational system where the communication channel user interface (UI) is just text messages, exchanged on chat-based platforms (as instant-messaging mobile apps).
Especially in academic parlance, the term conversational agent is synonym of chatbot, and the reason emphasis on the "agent" is maybe because conversational agents was considered software agents[2], special application of intelligent systems[3] and Multi-agent systems[4] research.
The term agent is so probably misleading, because assimilating any conversational system as a software agent that mediate between an end-user (e.g. a consumer of a business platform) and a business human operator counterpart. That use case is a specific use case and not always applicable.
The agentive role of a conversational system is commonly related to a the specifica scenario when the conversational system really acts as intermediator, requiring a final conversation between the end-user and a human operator (that's commonly called: human-in-the-loop).
But there are many cases where the conversational system is not properly an agent, by example in pure command-and-control conversational systems that just execute automation commands to action (electro-mechanical) actuators, by example in home-automation or industrial automation realms.