A barrier option is an option whose existence depends on whether the market rate touches a stated level before expiry. A knock-out ceases to exist if the level trades; a knock-in does not become an option until it does. The barrier is a term of the contract alongside the strike and the expiry, and it is what makes these contracts path-dependent: the same rate at expiry can produce two different outcomes depending on what happened on the way there.
Barriers change the shape of a contract's risk in a way that is easy to underestimate. A holder can be right about the direction and still finish with nothing, because the level was touched briefly at a moment of thin liquidity. What counts as a touch — which price source, which hours, whether a single print is enough — is defined in the contract terms, and those definitions are the whole of the argument when a dispute arises. Barrier structures are largely an institutional and structured-product market; whether anything of the kind is offered to a retail account is a question for the broker's own instrument list and documentation rather than an assumption.
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Guide
How the Options Market Reaches the Spot Price
Option positions have to be hedged, and hedging means trading spot. What that flow is, why it clusters near certain rates, and why almost everything said about it is an estimate.