General terms:
* Idea: A concept or suggestion formed in the mind.
* Conception: The act of forming or developing an idea.
* Cogitation: The act of thinking deeply or seriously.
* Meditation: Deep or prolonged thought.
* Reflection: Careful consideration of something.
* Contemplation: The act of thinking deeply about something.
* Notion: A belief, idea, or opinion.
* Opinion: A view or judgment formed about something.
* Belief: An acceptance that something is true or real.
* Assumption: Something taken for granted or accepted as true.
More specific terms:
* Imagination: The ability to form mental images or concepts.
* Intuition: The ability to understand something immediately, without conscious reasoning.
* Insight: The ability to understand something clearly and deeply.
* Reasoning: The process of thinking logically and forming conclusions.
* Judgment: The ability to form an opinion or make a decision.
* Memory: The ability to recall past experiences or information.
* Perception: The ability to become aware of something through the senses.
* Analysis: The process of examining something in detail.
* Synthesis: The process of combining different ideas or elements.
The best word to use will depend on the specific context and the nuance you want to convey.