A conceptual definition is the underlying understanding of something that is necessary to attain before understanding how it is used or applied. In science, it is necessary to understand the subject of research prior to conducting effective research.
The scientific process requires the testing of a hypothesis. A hypothesis is an idea about something which is formed based on observation or some sort of understanding about it and the desire to know more. In order to form a hypothesis and conduct subsequent testing, it is necessary to have a solid comprehension of its qualities and principles. This is conceptual knowledge.