Answering this question will put many more items on your list.
'Thing' is the most general physical object and any synonym might work in general (object, entity, etc). But it is also more specific than 'noun'.
If the word you have is a kind of thing, then you can always use a term that is more specific between 'thing' and more general than your new word. For example, among many things, a holster is: an accessory, a piece of equipment, a receptacle, a firearm holder, etc etc. For example, 'a holster is a holder for firearms that usually is worn hanging from a belt'). Another example is to follow the 'is a' relationship:
A dog is a canine is a carnivore is a mammal is an animal is an organism ...
But maybe your new word is not a thing. It could be a concept, like 'honesty' ("honesty is an abstract principle that favors answering with the truth") or a feeling like 'nausea' ("nausea is a symptom of intestinal distress that feels like you're about to vomit.").
At some point, there may not be enough single-word nouns to cover the entire range from general to specific, and then modifiers will be added:
Invisibility is a property, a property of substances, a visual property of substances, ...
This also goes for verbs or adjectives or whatever: "to run is to move you legs quickly", "pink is a color between white and red".
This successive refinement is what eventually ends in a complete definition of your word whatever it is.
What comes out of this is that you can make it more specific, but then you have more words to learn.