Both comprehension and understanding can be used to talk about someone's ability to understand something. Or about how something works: The meaning of understanding is a mental grasp :
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Understanding is a relation between the knower and an object of understanding. Understanding implies abilities and dispositions with respect to an object. Comprehension usually used with of.
Mental process of a person who comprehends;
How to use understanding in a sentence. Knowledge about a subject, situation, etc. He noted bond's apparent lack of.
