B737-800
Flight Crew Last 90 Days 0; Flight Crew Total 0; Flight Crew Type 0
Situational Awareness
Conflict NMAC
Flight Crew Took Evasive Action
Human Factors
Flying northwest along California-Nevada border near 38;000 ft. Pilot executed evasive maneuver to the right several seconds before I saw a white-topped 2-engine airliner fly to our left on the heading opposite to ours. I was seated on the left side of the aircraft; three rows from the rear. The other aircraft was in my field of view approximately one second. I was looking down; following terrain at the time. Horizontal separation was not zero; but may have been less than one wingspan. Vertical separation appeared to be about 400 or 500 ft.; if I assume that the other aircraft was also a 737-800. I spoke to a pilot very briefly to a pilot at the end of the flight; saying 'that was a close one; ...'. He responded with a four or five syllable response which I don't remember; but which was not a denial. I neglected to read his name tag or notice the number of stripes on the shoulder tabs.
A Passenger reported they observed an opposite direction aircraft in conflict with their flight.