A metal rod/pipe/mat/tape/wire/plate or piece of buried structural metalwork that is used to make direct contact with earth. The most common form of earth electrode is a copper rod that is driven into the ground.
You can not use utility pipes (gas/water/oil etc.) for this purpose as they may later be changed with plastic rendering your installation unsafe. You must still main bond these pipes but they are not to be relied upon for your sole means of earthing.