The Transfer of Property Act, 1882 defines 'sale' as the transfer of ownership of immovable property in exchange for a price, which must be fully paid or promised, with the transfer requiring a registered instrument for valuable property. Key elements for a valid sale include the competence of the seller and buyer, specific subject-matter that must be transferable, and an agreed price. The Act clarifies that a sale can occur through registration or delivery of possession and outlines conditions regarding the competency of parties and the characteristics of both tangible and intangible property.