The short answer
Using form, orientation, shading, insulation and ventilation to keep a building comfortable with minimal mechanical energy.
Passive design works with the local climate: blocking summer sun, admitting winter sun, crossing ventilation and using thermal mass where it helps. These decisions are cheapest at concept stage and nearly impossible to retrofit.
In hot climates, shading and reducing west-facing glass typically deliver more comfort per rupee than upgrading air-conditioning. Measure the effect with simple simulation before freezing the plan.
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