Order types, stops, post only, partial fills and the gap between the price you saw and the price you got.
Nothing matches that. Try a shorter word.
Most traders learn what a liquidation is by receiving one, and the description they carry afterwards is usually wrong in a way that matters. It is not the platform closing you at the price you expected. It is a mechanism designed to protect everybody else from your position, and understanding what it is protecting against explains every one of its features that looks unfair.
Prices are always relative. Nothing has a price on its own; it has a price in terms of something else, and that something else is chosen, usually by an interface rather than by the person trading. The choice determines what you pay, what depth you reach, and what your performance is measured against, and almost nobody notices making it.
The same asset trades at visibly different prices on different platforms, all day, in public. It looks like free money and it is one of the most reliable ways for a retail trader to lose some, because every element that makes the gap visible is also an element that makes it unavailable.
Almost every strategy performs worse live than it did in simulation, and the gap is usually attributed to bad luck or changed conditions. Both explanations are available and both are usually wrong: the gap has mechanical sources, and they can be listed.
Every trade needs somebody on the other side, and there are exactly two ways to arrange that. One matches you against another participant who chose their price. The other matches you against a formula that has no opinion at all. The difference determines your cost, your certainty, and which surprises are possible.
Once an order is large relative to what rests in the book, the biggest cost of the trade is the trade itself. Execution algorithms exist to manage that, they are simpler than their names suggest, and the version most traders actually need can be implemented by hand in an afternoon.
Traders argue about where stops belong as though it were a matter of judgement, and part of it is not. Three numbers describe any strategy, two of them are mechanically linked, and moving one moves the other in a direction that cancels most of the benefit. Knowing which part is arithmetic narrows the argument considerably.
The absence of a close is usually described as a freedom. It is more accurately a removal: the closing bell performed several functions nobody had to think about, and a market without one leaves each of those functions unperformed unless somebody rebuilds it on purpose.
Traders who optimise their fee tier to the last decimal often ignore a cost several times larger, because nobody sends them an invoice for it. Slippage is the gap between the price you decided on and the price you got. It has causes you can name, effects you can measure, and a component you can genuinely reduce.
Latency is the subject traders spend money on before measuring whether it is their problem. For a narrow set of strategies it is the whole business. For everybody else it is a small component of a cost dominated by two other things, and the money spent on it would have produced more elsewhere.
Liquidity is used as though it named a single quantity that a market has more or less of. It names three, they are measured differently, they fail at different moments, and a market that scores well on the one you looked at can be the one that costs you the most.
Most traders pick an order type by habit and discover its price afterwards, in the fill. Every type is the same trade in disguise: you are choosing between certainty of execution and certainty of price, and you cannot have both. What follows is what each choice costs, and where the cost hides.