Multi-tenancy is an architecture where multiple independent application instances operate in a shared environment with logical isolation. The article explores four database multi-tenancy approaches: single database with shared schema (lowest cost, highest tenant density but minimal isolation), single database with separate schemas (better isolation, more complex maintenance), database per tenant (highest isolation, most resource intensive), and multiple databases with shared schemas (balanced approach). Each pattern has distinct trade-offs between cost, isolation, maintenance complexity, and scalability that should guide architectural decisions based on application requirements.