Right definition of linear grammar I was referring book by Peter Linz, which defines linear grammar as follows:


  A linear grammar is a grammar in which at most one variable can occur on the right side of any production, without restriction on the position of this variable.


The wikipedia page also defines it similar way at the top of Linear grammar page


  a linear grammar is a context-free grammar that has at most one nonterminal in the right hand side of each of its productions.


By these definition, it seems that the production of the following form is allowed:


  $S\rightarrow aSb$


But next on the same page, wikipedia defines it as follows:


  linear grammars in which all nonterminals in right hand sides are at the left or right ends, but not necessarily all at the same end.


By this definition, it seems that the productions of the above form are not allowed.

Then what is correct definition?
They second quote from Wikipedia has lost its context. Here it is complete, with emphasis added:

Another special type of linear grammar is the following:

linear grammars in which all nonterminals in right hand sides are at the left or right ends, but not necessarily all at the same end.


A linear grammar has at most one non-terminal on the right-hand side of any rule, as per Linz' definition.
But any linear grammar can be transformed into an equivalent linear grammar of that special type, simply by adding a new non-terminal. Wikipedia goes on to show an example of this transformation. So it is possible to assume wolog that a linear grammar is in that form, if it turns out to be helpful.