I believe the colloquial definition has changed substantially over the last ~100 years in the US. As a concrete example, Travis Heights (part of Austin) was initially advertised as "Austin's first suburb", but is very much inside the city core today. In the UK, this is true of places like New Malden or even Wimbledon, which were as-built not part of London and were referred to as "suburbs", but are categorized that way by approximately no-one today.