Can anyone define for me what a “poser” is - at least in terms of how it is used on this forum?
A term used by a poor person who thinks they are fast to describe someone who owns a bike they can’t afford.
I resent that! I’m poor and I’m a poser! 
That’s what I thought.
Every sport/activity has posers. It’s someone that is drawn to the sport/activity (and/or emphasizes) for an aspect other than the sport itself. It might be flashy equipment, cool clothes, the image, the fellowship, etc.
Having posers is not tri-specific.
Signs of a poser (not tri-specific) … talks about their equipment more than their training (my barbell is tested for 1800 pounds, my 7-iron is made from …, on and on), more concerned with appearance/clothing than training times, goes to the gym to chat instead of lift, likes looking like a skateboarder more than they like to skateboard, has all of the equipment (weighlifting shoes, lifting gloves, elbow band, name-brand baggy pantsand tank, weight-belt … and one of the poundages … lifts like a little girl), etc. Essentialy, they do more acting than training. (IMO)
At this site, more often than not a poser is someone that rides an expensive bike but does not ride it all that fast. The ol’ “old guy in a new corvette” cliche.