reset

%reset:
    Resets the namespace by removing all names defined by the user, if
    called without arguments, or by removing some types of objects, such
    as everything currently in IPython's In[] and Out[] containers.

You can %reset all assigned variables in your session without confirmation by passing in the -f parameter:

%reset -f

Keep in mind this will also delete your %history. To preserve your history, you can pass in -s for a “soft” reset:

%reset -s

You can also reset just the input or output history or directory history:

%reset in

%reset out

%reset dhist