Defensive shields are physical protective barriers that enclose around the viewing screen and possibly the exterior of Core command on a battlestar.
Commander Adama orders activation of these shields as they fight off the first waves of a Cylon Raider attack on Galactica and her sister battlestars in the opening salvos of the ill-fated Battle of Cimtar (Saga of a Star World). Adama's commands to activate and deactivate the shield are "Positive shields" and "Negative shields", respectively.
Commander Cain also has the shields activated once the basestars began hitting Pegasus during the Battle of Gamoray He simply ordered "shield", which corresponds to the button that activates the barriers (The Living Legend, Part II).
Notes
- The physical properties, energy properties, or overall protection given by these shields are never explained in the Original Series.
- In comparison, Galactica's counterpart in the Re-imagined Series uses heavy armoring and ribbing through out its superstructure, enabling it to withstand a direct nuclear missile strike to its hull.