Wiki-Bullion

Of the metals that make up a laminated knife, this refers to the soft metal that is sandwiched between the hard metal. The presence of the base metal increases the overall durability of the knife because the softer metal absorbs the impact transmitted from the cutting edge.

Related websites
The Art of Making Japanese Knives: Detailed Process | KIREAJI
Japanese Knife Structures: Single-Edged vs. Double-Edged | KIREAJI