A "booster" in the context of mortgages refers to an external support mechanism, often provided by family members, to help a borrower qualify for a mortgage or secure better terms. This support can come in various forms, such as monetary gifts for a deposit, acting as a guarantor, or depositing funds in a linked savings account as security for the mortgage. Mortgage products that incorporate such support mechanisms are designed to help borrowers, especially first-time buyers, who might find it challenging to meet traditional mortgage criteria or save for a substantial deposit.