1.
knee
The joint in the middle of the leg.
2.
lap
The upper part of the legs when sitting, where someone might sit or place things.
膝 means both 'knee' and 'lap.' The expression 膝を付く means 'to kneel.' 膝枕 (hizamakura) is when someone rests their head on another person's lap. Knee problems (膝が悪い) are common topics in conversation, especially among older people.