Device that, alone, cannot prevent flow from one side to the other side of itself.
Source: API RP 17G, Recommended Practice for Completion/Workover Risers, Second Edition, July 2006 (Reaffirmed April 2011). Global Standards
Barrier (Barrier Element)
A component or practice that contributes to the total system reliability by preventing liquid or gas flow if properly installed.
Source: API STD 65 – Part 2, Isolating Potential Flow Zones During Well Construction, Upstream Segment, Second Edition, December 2010. Global Standards