Multi-tenant architecture means one software deployment serves many customer organizations (“tenants”), each with logical isolation of their data. The opposite is single-tenant, where each customer gets a dedicated deployment.
Most SaaS customer portals are multi-tenant — it’s what makes the per-user pricing model economically viable. Done well, multi-tenant architecture is invisible to the user: their data is isolated, their branding is custom, their permissions are scoped. Done poorly, multi-tenant systems leak data across tenant boundaries — a serious security incident category.
For B2B customer portals serving large enterprise customers with strict data isolation requirements, some vendors offer single-tenant or “dedicated instance” deployments at higher cost.