Now about "introduction and definitions".
Unless you really cannot do this, the introduction should in the ideal case be more accessible (say, understandable not only to an absolute specialist in your subfield). Even more importantly, the introduction should motivate the problem and present an informal overview.
This is introduction:
The order of the sepulation of sepulka is a well-known and important problem in Sepulka Sciences. We suggest an ordering of sepulation that increases the performance of almost each sepulka bundle.
This is main paper text:
Let ∆ be the main sepulation index of the current sepulka bundle, orderded lexicographically [Lem, 1957]. We denote with ∂µ the difference of sepulation gradient for...