What would be a a linguistic term for those nouns ending with -ing?
Examples: building, scaffolding, ending.
What are some other examples, and what do they all share in common semantically?
The wikipedia article on Gerunds has a decent section on the nominal and verbal properties of gerunds.
Based on feedback I'll point out a specific section of that Wikipedia article.
So from your examples, the grammatical term you might be looking for is Deverbal Nouns. Only a subset of deverbal nouns end in -ing, however.
I differ somewhat with @ghoppe on this issue. A gerund is still a verb that is being used as a noun with the addition of -ing. But words like "scaffold" are not verbs at all (except as English permits nouns to be used as verbs). So that fails the gerund "sniff" test for me.
If we look at Webster's 3rd New International Dictionary for -ing, we find this tidbit:
So while the -ing ending originated as a participial ending and later becoming a gerundial suffix, calling all words ending in -ing gerunds is a bit overstating the case. The fact that nouns are formed from other parts of speech by taking -ing endings would seem to eliminate gerund as a catch-all name for this kind of word.
If they genuinely sit within a phrase permitting some of the syntax of "normal" verb phrases, then they are are termed gerunds. As has been commented, this doesn't apply to any old word ending in -ing. A gerundive structure implies certain properties, such as allowing adverbs that you'd expect in a "normal" verb phrase. Contrast the reversal in grammaticality between the following: the adverb, not the adjective, is used with a "genuine" gerund, whereas the adjective and not the adverb occurs with an 'ordinary' noun:
1(a) *I saw this unexpectedly scaffolding.
(b) I saw this unexpected scaffolding.
2 (a) I saw them unexpectedly scaffolding the house.
(b) *I saw them unexpected scaffolding the house.
Otherwise, it's really common-- even in linguistics literature-- to just call them -ing forms. You don't always need a fancy word for things...!
(Other words like nominalisation don't refer specifically to -ing forms.)
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