In MTG: Arena I've built a Rakdos deck with the main strategy of making my opponent discard one or more cards—each turn if I'm lucky. I'm using cards such as Burglar Rat, Fell Specter, Davriel, Rogue Shadowmage and Carnival // Carnage.
Decks that let opponents put the top cards of their library into their graveyard, are often referred to as 'mill decks' or 'milling'. Is my deck a type of milling, or does it have its own term?
First, I want to note that "mill" as a deck strategy name is just named after "mill", which is the common unofficial name for the action of putting cards into the graveyard from the library. Following that pattern, the name of a discard strategy would just be "discard"
Discard alone generally isn't a strategy because it doesn't actually win you the game. Once your opponent's hand is empty, they can usually play the card they draw each turn before you have a chance to make them discard it.
Decks built around effects like Davriel's triggered ability can be called "rack decks", after the original card with that kind of effect, The Rack, though that fits better with decks that actually play that card. For example, 8-Rack is a commonly recognized name of a deck archetype that plays 8 cards with that kind of effect.
Is my deck a type of milling, or does it have its own term?
Some Google search (here also here) seem to indicate that the term used is "Discard Deck"...
Those terms are more informal than official. For example, the term "mill" historically comes from the card Millstone, which had an effect of putting cards from the library into the graveyard (as explained in this MTG article).
As you see, it was an informal term that rose from such card... as your deck's strategy is to discard it would follow that it is called a "Discard Deck".
Not really, but then making your enemy discard isn't generally a discrete strategy in MTG. It helps you win by denying them options, but doesn't actually move directly towards a win condition itself like Mill (drawing from an empty deck) or aggro (lots of direct combat damage). Generally discard options form part of a strategy, not the whole strategy. Generally decks that focus on discard use effects that punish it, or less likely lets them benefit from it. For example:
Some decks that focus on discard this way also speed the game up by running added draw, cards like Howling Mine, sometimes with the extra draw forcing more discard too, with Anvil of Bogardan, making sure there are cards there to discard, since if you focus too much on discard, a player ends up with no cards in hand and is able to play the one card they draw each turn pretty consistently while your discard spells are just dead in your hand. Drawing can be punished in these decks too, with cards like Underworld Dreams.
