Final answer:
A Baker's cyst is the accumulation of synovial fluid behind the knee, not related to tendonitis or bursitis directly but may present with similar symptoms like pain and restricted movement.
Step-by-step explanation:
A Baker's cyst is a condition characterized by a buildup of synovial fluid behind the knee. This fluid accumulates in a sac called a bursa, which is a fluid-filled space mostly located near joints to prevent friction between bones and other tissues. While tendonitis involves the inflammation of a tendon, and bursitis is the inflammation of a bursa, a Baker's cyst specifically refers to the accumulation of excess synovial fluid leading to a bulge and a feeling of tightness behind the knee, which can lead to pain and limited movement. Treatments may include medication, physical therapy, or in some cases, a procedure to drain the fluid.