I'm going to say a headwind is similar to Zwift/indoor training or climbing. Along with all of the fun physics stuff listed above - its the lack of "micro" breaks you get when riding with a tailwind. A headwind is constant steady resistance, always forcing you to keep momentum, forcing you to keep steady power on the pedals.

I often find it easier to maintain steady state power into a headwind (or on a climb) than I do with a tailwind.

My main route, a path along a canal that connects two large bays (Chesapeake and Delaware bays) is a funnel for wind. It's either a headwind or tailwind if the wind is coming from all but due North. Every other wind get's funneled down the canal. Winds out of the South cause a tailwind/headwind/tailwind/headwind situation - as the winds come up the bays and meet at the midpoint of the canal.

What's fairly interesting, no matter the wind direction or speed - at a constant power level, my ride times are within a few min of each other over the 30 mile out and back.