“Altcoin” aka alternative coin, is a term primarily used by Bitcoin maximalists, who think that it is superior to all other cryptocurrencies. They use this term to denote any and all cryptocurrencies that are not BTC.
Cryptoassets are assets in the form of a digital token or digital coin, secured by cryptography and built on blockchain technology.
The term refers to the token itself rather than the software upon which it is built. For example, Ethereum’s cryptoasset is ether (ETH).
In some cases, including Bitcoin, the software and the cryptoasset bear the same name — for those currencies, you can distinguish between them because the name of the software is capitalized (“Bitcoin”) while the cryptoasset is written in all lowercase or abbreviation caps (“bitcoin,” or “BTC”).
Fiat refers to government-issued currencies, such as the U.S. dollar, the euro, and the Brazilian Real. They do contrast with cryptocurrencies, but these two are slowly beginning to blur as governments and world economies begin to issue and adopt stable coins and other cryptocurrencies into their financial systems.
Stable coins are cryptocurrencies intended to maintain a stable price and value, in contrast to the other extremely volatile cryptocurrencies (such as Bitcoin and Ether). Financial crypto institutions achieve this goal by issuing tokens with a value pegged to particular fiat currencies.
There are decentralized and centralized types of stable coins.
For example, the MakerDao foundation is a decentralized project that issues a token, DAI, which is designed to always be worth $1 USD. DAI is a stable coin pegged to the US dollar and backed by Ethereum as collateral.
And Tether is an example of a centralized, private project that issues a token, the USDT, which is a stable coin also pegged to the US dollar, but backed by fiat and other mixed assets.