I'm looking for a word to describe something that can be physically passed through like a ghost, mist, or a gas. Initially, I was thinking of the work transparent, but I feel like this is better used to describe something that is see through.
Borrowing from Carl Sagan's The Dragon in My Garage, which suggests this word for specters:
incorporeal - having no physical body or form
(Definition from Merriam-Webster.)
I have to take issue with permeable. A sieve is permeable, a sponge is permeable, but unless you're a molecule or a virus or something similarly small you can't pass through.
How about ethereal or penetrable?
ethereal: lacking material substance
penetrable: allowing someone or something to pass through or enter : able to be penetrated
It's not hard to imagine passing through anything ethereal.
It would not be unreasonable to consider a ghost, gas, or mist penetrable, in fact infinitely so.
Note: @Drew mentioned penetrable in a comment.
The word you're looking for is permeable.
Ghosts are permeable, a gauze curtain is permeable, a cloud is permeable.
allowing liquids or gases to pass through
Permeable is a good word. But if you're looking for variety, I'd suggest intangible. In comics, some suprheroes(e.g. Martian Manhunter) have this super-power where objects and people can pass though them as if they don't exist, helping them to dodge bullets etc. It's referred to as Intangibility.
1. unable to be touched; not having physical presence. "the moonlight made things seem intangible"
If the context is ghosts or other spiritual entities, you can also use words like spectral or incorporeal to convey the same meaning.
To expand on @stevesliva answer:
- Having no material body or form.
As an IT guy, I feel compelled to add traversable
capable of being traversed
traverse: a. To travel or pass across, over, or through: a ship traversing a channel; light traversing a window.
Traversing is more broad than "passing through". Even if maybe it's not the exact meaning you are looking for, sometimes it can be the most appropiate word.
fluid: it means gas or liquid.
"a substance that has no fixed shape and yields easily to external pressure; a gas or (especially) a liquid."
One other way you can reference such an object is as "medium":
The intervening substance through which sensory impressions are conveyed or physical forces are transmitted
You could use the word transpirable
This is defined by Dictionary.com as:
To emit or give off waste matter, watery vapor, etc., through the surface, as of the body or of leaves.
To escape, as moisture or odor (or "gas"), through or as if through pores.
Alternatively, you could use respirable (regarding ghosts or mists), as defined by the Merriam-Webster:
Fit for breathing; also: capable of being taken in by breathing.
Hope this helps :)