The single unit of an independent cryptocurrency that is used to exchange value much like the word dollar is used in the United States to describe a single unit of currency. Often the terms “coin” and “token” are viewed as synonyms. However, there is a difference: a coin refers to a single unit of a cryptocurrency such as Bitcoin which functions to transfer value using its mainnet whereas a token is a programmable application created on top of a cryptocurrency's mainnet. Ethereum's mainnet allows for the creation of tokens using its ERC-20 standard.
A currency pair is a denomination a particular asset is valued. Currency pairs apply to forex, stocks, commodities assets and other financial instruments. Cryptocurrencies are priced in fiat or other digital assets with large market capitalization such as Bitcoin and Ethereum.
Decoupling, as it pertains to cryptocurrency markets, is when the price of an altcoin no longer follows the price trend of bitcoin. Historically the price of an altcoin was tether to that of bitcoin because many exchanges only offered bitcoin/altcoin trading couples. As the market matures and currencies such as Ethereum and the USD start coupling more with altcoins, a shift in demand may lead to the loss of price correlation between Bitcoin and various altcoins, for instance: during mid to late 2018 the price of Ether in USD no longer followed Bitcoin's price trend.
Double spending is the act of spending the exact same unit(s) of a currency twice which has been a driving factor in currency's evolution. Bitcoin and blockchain technology was originally created to be a mechanism for “solving the double spending problem” digitally using advanced peer-to-peer cryptographic technology.