documentclass{standalone}
usepackage{tikz}
usepackage{pgfplots}
begin{document}
  deflabels{xlabel=x, ylabel=y}
  begin{tikzpicture}
    begin{axis}[labels]
    end{axis}
  end{tikzpicture}
end{document}

...
    begin{axis}[xlabel=textit{x}, ylabel=y]
    end{axis}
...

documentclass{standalone}
usepackage{tikz}
usepackage{pgfplots}
begin{document}
  deflabels{xlabel=textit{x}, ylabel=y}
  begin{tikzpicture}
    begin{axis}[labels]
    end{axis}
  end{tikzpicture}
end{document}