By tradition, a popular vote victory of 55% or more is considered a landslide. The last popular vote figure I saw was 57%, which qualifies this as a landslide.
When a presidential candidate wins in a landslide and his party's numbers in Congress increase, he is considered to possess a mandate.
Obama's mantra for the campaign was "Change". I have no idea what he plans to do with his mandate, but he has one, and he has the congressional backing to get it enacted.
Sorry, but I must disagree. The last popular-vote percentage I saw for Obama was 53% - not 57%. Also, it appears that the Democrats will not have a fillibuster-proof majority in the Senate, so the worst of their expected legislative proposals will not be enacted (at least if the Republican leadership holds its members together).