Is it a function of grade or grade over distance or something else?

Inquiring minds.

It becomes a climb the instant you start pedalling up the grade, no matter the grade over distance! That's my opinion, anyway!

When I am on the verge of being dropped.

As soon as you start changing to a lower gear- You are on a climb. To be a slope though is a different matter- and as for Hills_____________When do they turn into Mountains?

I don't think there is any technical quantitative specification. Basically, it's a perceptual thing. You think it's flat, it looks flat, yet your going pretty slow compared to what you're use to for how hard you're pedaling. Going the other way, you think it's flat, it looks flat, yet your going significantly faster than you would on a truly flat road.

This road appeared to be flat for 26 miles.
Looking back you could tell, but not while riding forward.

SKT, I suppose a false flat is technically a climb or a decent, but I still consider it the same as riding on level ground.

Brad

Go riding through Skyline Drive and Blue Ridge Parkway and you'll see some false flats.

I'm thinking that as long as you're sitting in the saddle and spinning away, it's a false flat. When you start standing on the pedals, lowering your cadence and grinding away, it's a climb.

A false flat is simply an optical illusion. There are different variations of these illusions. Here's one that many perceive as going up hill, but actually they are going downhill https://www.youtube.com/watch?v=RJYuF2sYzGw

As soon as I have the need to downshift

This is assuming there isnt a sudden change in wind direction.

When your heart rate monitor starts to beep "over-range."

+1 As soon as I have to downshift.

When it hurts.

Hmmm is this a trick or something? A "false flat" can not be flat or it wouldn't be false now would it? Therefore it has to be an elevation increase. In other words, it is a climb. It might not be much of a climb. It might be what I would call a "rise". But it is just a matter of semantics.

I would just as soon avoid a discussion on the exact definition of what consitutes a climb. I mean we would have to hash out the minimum percent grade involved and the minimum elevation increase involved. Personally, I think "climbs" are in the eye of the beholder. If you think that it is a climb, it is.

Here's a pretty good definition of a false flat; take note of the last paragraph.
https://www.fitsugar.com/What-False-Flat-8216271
"You know that sensation when you're on a stretch of road, either running or cycling, and it looks flat but you're still huffing and puffing? When you look at the road or trail straight on there is no perceptible rise in the terrain, yet you're working extra hard to maintain your speed? That, my dear, is a false flat, and the roads in my neck of the woods are full of them. The slow creep of roads away from the San Francisco Bay offer many almost imperceptible uphill grades, but I have felt them elsewhere. False flats are deceptive and can be soul crushing; unlike discernible hills, there is no victorious summit creating a sense of accomplishment and a bit of glorious descent. One biking forum explained that the grade of these ascents as only being one to two percent, but that is enough to feel it on the climb up.

Coming down a false flat on the other hand makes you feel quite speedy, but rarely will one attribute their sudden speed to the downhill false flat. The term seems mostly used to explain the difficult, if imperceptible climbs."

False flat is just a name someone came up with to describe the perception of flat road when it is actually a grade.
One time I was in a group going up a slight grade along a creek. Vallygirl was there and she commented that the creek was running "the wrong way".

Did you ever see the movie "The Englishman who went up a hill and came down a mountain"?

Thanks, all, for your perspectives. I got curious about this yesterday... we have a number of these kinds of roads in town.
I look straight ahead and it's just a flat road. I look off to the side and it shows a pretty decent slant.
I checked my Garmin -yeah, I know- and it says it's around 3%
So, I got to thinking... is this a climb?

Something similar happens on a "real" (>6%) climb... look straight ahead and you feel like you're crawling.
Look to the side and you can see you're actually moving at a pretty good clip... and I only climb at around 6-8 mph

I think this is a philosophical question/issue, something akin to:

I pedal, therefore I am.

When we were kids, we would periodically go visit what we called "gravity hill". You could stop your car or bike, let it coast, and swear you were watching it roll uphill.

I call the next step up from a false flat an "incline".

Yeah, that's what is happening here, but instead of calling it "gravity hill" they credit it to ghosts. https://www.youtube.com/watch?v=jDeSS9isS9c&feature=fvsr

Off topic, but I just want to say I'd kill for a shoulder that wide, even in that treeless expanse!

Especially one like that with no rumble strips.

4 years ago and I climbed Ventoux. Not so much difficult but it was one continual hard slog in 28/28. Got to the cafe about 2/3rds up and you turn right up to a left hairpin.

Now remember that I was tired- legs were aching and I was in the lowest gear on the bike from about 1/2 mile from the bottom with a cadence of around70. First thing I noticed was that cadence went up to around 85 so I changed up a gear then another. False Flat? Noway- it was just that the 8% slope had turned into a 5%.