Masdars (was: Verbal noun, verbnoun, deverbal noun, gerund, infinitive)

What are masdars?
Which languages have them?
How do they differ from other verbal nouns?
Do any linguists refer to them by another term?
Do they ever get called "infinitives" or "gerunds" or "gerundives" or "supines"?
What's the "best" term for them?
Who calls them masdars and who calls them something else?