In one sentence
A mortgage secured by a tenant's leasehold interest rather than a fee ownership of real property.
The longer version
When the borrower owns a lease rather than the land, the lender's collateral is the leasehold. Leasehold mortgages require the underlying lease to permit financing and to give the lender notice and cure rights if the tenant defaults.
The lender's whole position depends on the lease surviving, so ground lessor consents and estoppels are central diligence items.
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