* φαντάζομαι (phantázōmai): This is probably the closest single word. It carries the sense of "to appear to the mind," "to seem," or "to have a vision," often implying a visual image, but also capable of encompassing other senses.
* ἐννοῶ (ennoô): This means "to have in mind," "to conceive," or "to understand." It suggests a more intellectual form of imagining, focusing on the conceptual rather than the visual.
* διανοοῦμαι (dianooumai): Similar to ἐννοῶ, this means "to think," "to consider," or "to reflect." Again, more intellectual than purely visual imagining.
* ὑπολαμβάνω (hypolambánō): This translates to "to assume," "to suppose," or "to conjecture." It's suitable when the imagining is a form of speculation or guesswork.
The best choice would depend on the context. For a purely visual imagination, φαντάζομαι is likely best. For a more abstract or conceptual imagining, ἐννοῶ or διανοοῦμαι might be more appropriate.