There are certain laws pertaining to a floor. For example, if a Sefer Torah is dropped on the floor, one is supposed to do some sort of atonement (eg., most famously, fasting for 40 days).
What is considered a floor in Halachah? Does an elevated platform have that status? What size? How about a stage - or a Bimah, for that matter? Is a car floor a floor? A bus, train, or airplane? Moving or parked?
In the Sefer Avnei Yashfei 4:109:2 was asked if a sefer Torah fell inside the Aron Kodesh does one have to fast.
He writes that one does not have to fast(there is more savoros but put in whats applicable here), the main reason being that it is not a place for walking like the Atzei HaLevanon 2:71 writes(he is quoted in previous part of tshuva regarding some of the sefer Torah fell,and some fell on a bench),He asked Rav Elyashiv and he held the fact that its not for walking it is not like falling,and he added that there are some who are even meikeil in regards to steps to the aron kodesh because its main purpose is not for regular walking.
It seems that floor means a place that is made for walking.
The Magen Avraham (44, 5) refers to the custom of fasting if a sefer torah or tefillin fall onto the ground. He does indeed use the words 'al haaretz'. (Seemingly the only difference between sefer torah or tefillin in this law is that one fasts for a sefer torah falling even if it was in its wrapping/container.) The Mishna Berura cites the Magen Avraham (40, 3.)
Piskei Teshuvot (vol 1 p.359) refers to Shut Afarkasta De'aniya that who says that this only applies if the tefillin fell onto the actual ground, however, if they fell onto a vessel that was on the ground one would not have to fast.
The Kaf Hachayim (40, 7) based upon the Ben Ish Chai does not rule that they have to fall onto the ground. If they fall more than three tefachim it is a denigration of the tefillin and one should fast/give charity. It is not important what they fell onto but rather that they fell from his hand.
According to the first opinion it is clear that something like a bimah is not considered part of the floor. At the very least it would be a vessel upon the floor.
Generally, there is a law of lavud, i.e. anything within 3 tefachim of the ground is considered part of the ground. For example, the Shulchan Aruch discusses this regarding praying on a raised area above 3 tefachim (90, 1). However, if the raised area is 4 x 4 amos wide it has a status as a separate floor (90, 2). The cases you mentioned of cars, trains and planes the floor would usually have the dimensions to be considered a floor in these regards.
I don't believe that there's a 'one size fits all' answer to the more general question of how we define a floor in halakha, but we may be able to extrapolate a few principles.
We can import a halakha from the laws of shabbos (and sukka): levud. This means that anything withing 3 tefachim is considered attached. If a step etc. is raised slightly off the ground, but less than 3 tefachim, then it is not raised enough to be considered something other than floor.
One of the only cases where the Shulchan Aruch gives some parameters for what's considered a floor is in the laws of prayer. He writes in O.C 90:2 that a person may not daven on a raised platform, but if this platform is 4 by 4 amos, then it is permissible. One reason is because a person davening on such a platform won't fall off, but another reason (see Beis Yosef in the name of Mahari Abuhav) is because an area that is 4x4 amos is considered a 'reshus bifnei atzmo', it's own domain (also from Gemara Shabbos, see daf 5a). This reasoning sounds like it would be applicable anywhere else we need to define a floor: something that is 4x4 amos is its own floor, even if it's a raised platform.
However, I'd imagine that another requirement to be a floor is that it's actually used as a floor. A table can also be 4x4 amos but is not a floor because nobody uses it as such. Especially for a halakha (not necessarily this one, just hypothetically) which depends on the disrespect that is conveyed by a floor, I'd imagine that only a platform that is made to be used primarily for being stepped on fits the bill.
the floor isn't relevant as far as fasting goes, one would fast no mater if the sefer Torah was dropped on the floor or on a table. As quoted from the kitzur shulchan aruch (chapter 28 paragraph 12), "If a sefer Torah falls out of someone's hand, even if it is [covered] by its mantle, he must fast, and it is the custom that those who see it [fall] also fast." Here the emphasis is on the falling and not on where the Torah fell.
Most of those examples would indeed be considered a floor (with the exception of a Bimah which would be like a table). However, depending on the situation, different halachot may apply. For example, in some situations it makes a difference if the floor was carpeted etc. In other words, if heaven forbid this situation would arise and either Tefilin or a sefer Torah were to fall onto the floor, it would be best to ask a Rabbi about that specific situation.
