Junk dimension: A junk dimension is a collection of random 
transactional codes, flags and/or text attributes that are 
unrelated to any particular dimension.  Simply to say a 
randomly used dimension is junk dimension

What shree uday said that is right and with that there is
one another thing we can say ........
i.e we can simply say which dimension is having low cardinality

A number of very small dimensions might be lumped together 
to form a single dimension, a junk dimension - the 
attributes are not closely related. Grouping of Random 
flags and text Attributes in a dimension and moving them to 
a separate sub dimension is known as junk dimension.