Is this lump located at the level of joint line?
Have you noticed that lump changes in size with knee flexion/extension?

The lump seems to my untrained eye to be at the middle of the joint line, but off to the inside. It does seem to protrude more or less with flexion and extension. I guess you could say it "hides" a bit when I extend my leg. But I can always see it. My husband thinks it seems to be attached to the tibia.

Thank you for asking.

Is there some pain and swelling?
Have you experienced a trauma or chronic infection of your knee joint?

I have very little pain -- an occasional mild ache or twinging pain. I've never had knee problems or trauma.

Pain and swelling or only focal mass at the level of the joint, and mass that change in size with knee flexion/extension is suggestible for meniscal cyst.
Can, you, visit your doctor and ask him for refferal for some imaging techique (if she/he suspects the same)?
MRI is usually the choice for diagnosing meniscal cyst.