All

All allows selection along a dimension in Part.

Attributes[All] := {Protected}

Simple examples

In[1]:= mat = Table[Table[a*b^2, {b, 5}], {a, 5}]
Out[1]= {{1, 4, 9, 16, 25}, {2, 8, 18, 32, 50}, {3, 12, 27, 48, 75}, {4, 16, 36, 64, 100}, {5, 20, 45, 80, 125}}

Use All to select along the entire dimension:

In[2]:= mat[[5,All]]
Out[2]= {5, 20, 45, 80, 125}