Sometimes called first charge. The earliest (in history) mortgage recorded against a property, given by its current owners. Most high street building societies only accept first mortgages.
Sometimes there is more than one mortgage on a property. In the event of a default by the borrower, the holder of the first mortgage will be repaid first by the proceeds of a sale. If there is a second mortgage holder, they will be paid with any remaining proceeds. This also creates a lien, which has first priority over all other voluntary liens.