A read-only counter tracking successfully transmitted frames on this interface that were delayed by exactly one collision before getting through. This is a normal, expected occurrence on shared/half-duplex Ethernet segments, and tracking it separately from multiple-collision frames lets an admin gauge how congested the collision domain is; a frame counted here is also counted in the standard ifOutUcastPkts/ifOutNUcastPkts counters, but never double-counted in etherTxMultipleCollisionFrames. A moderate, steady rate of single-collision frames on a half-duplex hub-connected port is normal, but a sharp rise suggests increasing traffic load on that shared segment.