Solid bridging

Solid bridging

A form of lateral stiffening between timber joists to prevent them from twisting, and to provide a fire stop. Short lengths of wood similar in depth are fixed between each adjacent pair of joists.
Illustrated Dictionary of Architecture Copyright © 2012, 2002, 1998 by The McGraw-Hill Companies, Inc. All rights reserved

block bridging, solid bridging, solid strutting

block bridging
Short members (boards) which are fixed vertically between floor joists to stiffen the joists.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.