What is a blend?

As Led Zepplin once said "sometimes words have two meanings".  That seems to be the case with blended whiskey.  Take, for example, Johnny Walker.  It is a blend.  But it is a blend of (depending on the label) around 40 different malt whiskies aged in oak for around a dozen years.  Other blends, not so much.  I haven't found a recent bottling that displays the breakdown, but I did recently come across a bottle of Hannah and Hogg blended whiskey that displayed the breakdown on the label.  I believe it was 20% straight whiskey aged 4 years and 80% grain neutral spirits.  Often I will read whiskey reviews that assumes any blend, especially American whiskey, is a blend of grain neutral spirits and straight whiskey.  But since the labeling doesn't tell you, who really knows?  I sure don't.  So what does blended whiskey mean?  I guess only what the label or the internet tells you.