I feel like Rosencrantz or possibly Guildenstern asking this but…
I was boiling water but had to run before it could finish. I turned off the heat and noticed that steam began to rise from the water. I turned the heat source back on and poof, no more steam. Turned back off (really needed to go at this point) and again steam appeared. Is there a name for this and why does it happen?
The visibility of the steam is not a reliable indicator of the amount of water vapour in the air; when you turned off the hob, the air surrounding the column of rising water vapour would be cooler and it becomes more visible (like your breath does on a cold day).
“Heat source?” Why the big secret? Could you possibly be using a gas stove? Where hot gasses from below the pot would travel up its outer walls and then form a vertical column, where they would mix with the rising water vapor from the water?
Just a bit of a nitpick…
Steam is invisible. If you see water vapor…that’s just what it is.
Yep, if you see it, it ain’t steam. Steam is invisible.
From www.dictionary.com :
The etymology traces back to Middle English and Old English. I doubt whoever named steam originally said “That’s steam. Not the stuff you see, the invisible stuff.”
Redefining words so precisely that the original (usually more colloquial) meaning is excluded has been a peeve of mine for a while.
The popular usage of “steam” is synonymous with “mist.” In fact, so many people call mist “steam” that it has more-or-less been redefined. But if you look in any dictionary, including dictionary.com, you’ll find that the first (and primary, and “official”) definition is that it is “the vapor phase of water” or “water in the state of vapor.”
And, lest the point be missed in the etymological quibbling …
In an environment which is at a temperature of over 100 degrees Celsius (at 1 atmosphere of pressure, naturally), you won’t see any water vapor. The clouds of ‘steam’ are a result of the water vapor cooling back into its liquid form, in droplets tiny enough to be suspended in the air. When the temperature near the pot increases (when the burner is turned higher), that effect will no longer be seen in that area …
So you could say that it is “water vapor”.
So what you’re saying is: Steam is invisible. If you see steam… that’s just what it is. 
Seems that it’s not “steam” that is missunderstood, but “water vapor”, or both really, given that they are synonomous.
Well as long as no one gets steamed up over it . . .
Unless the meaning is labeled “slang”, all the definitions are “official”. Your and Reeder’s comments imply that using “steam” to indicate the visible mist is somehow incorrect, and that zber is misusing the word when in fact he/she isn’t. Look at this entry for the word “hand”. There are 19 definitions for the noun “hand”, many of which have subdefinitions. Are all but definition 1a “unofficial”? If I refered to being dealt a hand in a thread about poker, would you pop in and say "‘hand’ means “The terminal part of the human arm”?
Do you have a cite for this? My expectation is that the word “steam” in Old English refered to what was seen when water was boiled. In other words, that using “steam” to refer to only the invisible gaseous water is the more recent definition.