core

noun:the tough central part of various fruits, such as an apple or pear, containing the seeds
- He ate the apple down to the core and then threw it away.
noun:the central or most important part of something
- The core of the problem is a lack of funding.
noun:a group of muscles in the torso that stabilize the spine and pelvis
- Pilates is an excellent way to strengthen your core.
adjective:of central importance; fundamental
- These are the core principles of our organization.