Enfilading Fire: The act of firing across the longest axis of an enemy unit or formation, allowing for maximum casualties. Can be either flanking fire or frontal fire, depending on the enemy formation.
Direct Fire: Artillery term. Fire where the gunners can directly see the fall of shot. More accurate, but puts guns at greater risk.
Indirect Fire: Artillery term. Preferred method during WWI. Fire where the gunners cannot see the fall of shot, but are directed via forward observers and grid coordinates. Less accurate, but keeps guns concealed.
PC: Abbreviation for "poste de commandement", the French phrase for "command post". Was used by the AEF.
Fire and Maneuver: Tactic used where part of a unit lays down covering fire while the other part moves. Units advance by alternately moving and covering each other.
Courtesy of Global Security
Effects of Enfilading Fire during the Dieppe Raid in WWII. Machine gun bunker visible just right of the standing soldier's head (Courtesy of Wikipedia)