Penetration

(1) The depth below the surface of the base metal to which welding heat is sufficient for the metal to melt and become liquid or semi-liquid. Also called the depth of fusion. (2) The ability of arc or electrode to reach into the root of the groove between two members being welded.

Related Links

Penetration | Definition of Penetration
Penetration
Urban Dictionary: Penetrating
Penetration | Definition of Penetration at Dictionary.com
PENETRATION | definition in the Cambridge English Dictionary