What does the n mean in n-butyl lithium?

It means the lithium is not attached at a terminating carbon, but at the second carbon, or the third carbon. Since carbon 2 and carbon 3 are equal in terms of position, it is called "n-butly" rather than "2-butyl".

e.g.

...H..Li.H..H

...|....|...|...|

H-C-C-C-C-H

...|....|...|...|

...H..H..H..H

Edit - to see why the 2 and 3 carbons are equal, simply rotate the above structure by 180 degrees and allow the carbon on which teh Lithium is attached to rotate by 180 degrees around the single bond, now it looks like it is attached to carbon 3, but it is actually carbon 2....

Normal, which means that the 4 carbon atoms are in a straight line, not branched.

n-butyl...means a 'normal' straight chain molecule, as opposed to iso- branch chain as in iso-butane (i-C4H10) (or methyl propane) for example and, cyclo- cyclic (ring) chain as in cyclo-butane for example (cy-C4H8).

normal