A valve with unobstructed flow area dimensionally equal to or greater than the nominal connection size.
Source: API STD 53, Blowout Prevention Equipment Systems for Drilling Wells, Upstream Segment, Fourth Edition, November 2012. Global Standards
Valve whose closure mechanism has a bore dimension the same as or larger than the valve body.
Source: API SPEC 6A, Specification for Wellhead and Christmas Tree Equipment, Twentieth Edition, October 2010 (Addendum November 2012). Global Standards