$HistoryLength

$HistoryLength
specifies the number of previous lines of input and output to keep in a Mathematica session.

Values of In[n] and Out[n] corresponding to lines before those kept are explicitly cleared.

Using smaller values of $HistoryLength can save substantial amounts of memory in a Mathematica session.

The default is to save the entire history:

Change the setting to save no history:

This means the previous input can no longer be used directly:

By assigning to a variable you can still refer to previous results:

The default is to save the entire history:

Out[1]=

Change the setting to save no history:

Out[1]=

Out[2]=

This means the previous input can no longer be used directly:

Out[3]=

By assigning to a variable you can still refer to previous results:

Out[4]=

Out[5]=