Medial

Medial: An anatomical direction that refers to a structure nearer to the midline of the body. For example, the sternum is medial to the pectoralis major.

• Note that "midline" in this definition refers to a line through the middle of the body when the body is facing front or back in the anatomical position. That is, a line that would bisect the body into left and right halves, also known as the mid-sagittal line. Medial would be closer to the mid-sagittal line.
  

The opposite of medial:

• Lateral

For more information on the anatomical directions, check out:

• Lesson 1: Anatomical Position and Anatomical Directions

Here is a list of anatomical structures with the word "lateral" included in their name:

• Medial Gastrocnemius: The medial head of the more superficial and two-headed muscle of the calf, known as the gastrocnemius . Note that there is also a "lateral head" of the gastrocnemius.
• Vastus Medialis: The most medial muscle of the quadriceps . This muscle may be visible as the "teardrop-shaped" muscle on the medial side of the knee.
• Medial Collateral Ligament (LCL): The medial collateral ligament aids in the stabilization of the "inside" of the knee and prevents the tibia from abducting in the frontal plane. Note, that the "lateral" collateral ligament stabilizes the "outside" of the knee.