Barrier to entry
A structural obstacle that makes it difficult or costly for a new competitor to enter a market: high initial capital requirements, economies of scale, patents, regulation, network effects or privileged access to distribution channels. The higher the barriers, the more defensible the market.
In practice
Barriers vary in nature. A regulatory licence — banking, insurance, pharmacy — is nearly insurmountable. A patent is time-limited (20 years) but solid. A strong brand with loyal customers forms a behavioural barrier that cannot be attacked on price alone. For investors, barrier height directly correlates with margin sustainability: markets without barriers see competition erode pricing down to marginal cost.