What term refers to an idea or situation based on an assumption or guess?

The term that accurately describes an idea or situation based on an assumption or guess is "hypothetical." This term is often used to describe scenarios that are not grounded in established facts or actual events but rather explore possibilities that could arise if certain conditions were met. A hypothetical situation allows for discussion and analysis of outcomes based on various premises without claiming them to be realities.

In different contexts, "speculative" can also refer to ideas based on conjecture, but it tends to imply a focus more on investment or risk-taking scenarios rather than the broader exploration of possibilities. "Theoretical" often relates to principles and concepts that are derived from theories rather than practical experiences. "Imaginary" refers to something that exists only in the mind and does not have a basis in reality, which doesn't quite capture the element of assumption present in hypothetical situations. Hypothetical is the most fitting term for an idea based specifically on an assumption or guess.