There's been some talk on and off again here on ET about Arbitrage. And from what I've seen, the term is frequently mangled.
Legitimate Arbitrage is as rare as hen's teeth - and disappears very quickly.
One example might be buying BTC on one exchange for $XYX and selling it two seconds later on another exchange for $XYZ + $20.
Usually what most people label as "arbitrage" is actually a highly correlated spread trade that someone is legging into and out of.
Even buying a physical commodity and selling the direct analog futures contract is not technically an arbitrage. And it isn't a direct arbitrage in reality. By definition an arbitrage is relatively risk-free and is exploiting a mismatch - usually along the lines of a communication lag or unequal and temporary market duplication typically with multiple marketplace locations.
In other words - very temporary market dislocations regarding the same product.
For example, buying an On-The-Run US Ten Year Note and selling the CBoT Ten Year Futures Contract has basis risk. The Repo market rates have been unusually volatile the past couple years.
Selling physical oil in Texas and buying the Nymex WTI Futures Contract has basis risk. Look at what happened to the May 2020 futures contract - the complete lack of storage compatible with the futures contract specifications and the amazing lack of demand from refineries led to the famous negative futures price.
One of the most important premise for arbitrage is that the two different products have to be extremely similar to the point that they are completely interchangeable, e.g. buy/sell a cross exchange rate vs. buy/sell one direct exchange rate to the common currency and then buy/sell to the other direct exchange rate to the same common currency right away with no carrying cost to take advantage of any price differentials. The currencies are exactly the same and can be obviously used interchangeably. Any time when you introduce other costs and extra fees that's levied on one product but not the other like carrying costs, storage costs, you change the product itself and it reduces the arbitragibility between the two.
e.g. if you can borrow cheaper than Fed Funds