Subordinate; incidental; secondary.
In Britain, a by-election is the election of an MP or local councillor to fill a vacancy arising during a term of office. A byroad or byway is a minor road or track; a by-catch is the unwanted fish and other marine creatures trapped by commercial fishing nets during fishing for a different species; a byname is a sobriquet or nickname, especially one given to distinguish a person from others with the same given name; a by-product is an incidental or secondary product; byplay is secondary or subsidiary action or involvement in a play or film.
In bypass, a road that passes by a congested area to provide an alternative route, or bystander, a person who is present at an event or incident but does not take part, the sense is that of ‘aside’. A by-law (also bye-law), a rule made by a company or society to control the actions of its members (in Britain, a regulation made by a local authority) may instead derive from Old Norse býjar, genitive singular of býr, town.