What is the meaning of the word "ground"?

Definitions:

a position to be won or defended in battle (or as if in battle) noun
- they gained ground step by step
a wall or canvas prepared to be painted on noun
- I was ready to begin painting the ground I had setup earlier.
bring to the ground verb
- the storm grounded the ship
confine or restrict to the ground verb
- After the accident, they grounded the plane and the pilot
connect to a ground verb
- ground the electrical connections for safety reasons
hit a groundball verb
- The batter grounded to second base.
the part of a scene or picture that lies behind objects in the foreground noun
- The artist posed the model against a background of rolling hills.