Scalable is an adjective applied to systems, businesses, processes, or technologies whose capacity to handle increasing load, demand, or scope can be expanded substantially without a proportional increase in cost, friction, or breakage.
In software, scalability is usually decomposed into vertical (adding more resources to a single machine) and horizontal (adding more machines), and is measured against latency, throughput, and cost-per-unit-of-work as load grows. In business, the term carries a related but looser sense: a model is "scalable" when growth in revenue does not require proportionate growth in headcount, fulfillment effort, or per-customer overhead. SaaS, marketplaces, and platform businesses tend to qualify; consulting, custom services, and labor-intensive operations usually do not.
The word has drifted into general usage as a vague positive — anything good is "scalable" — but the technical and economic meaning remains specific: a system is scalable only to the extent that the marginal cost of additional capacity is meaningfully lower than the marginal revenue or value it produces.