Label is long   text
lab2            blalballalaa
label3          possible second rule
label four      hello world

newenvironment{mydescription}[1]
  {begin{list}{}%
   {renewcommandmakelabel[1]{##1:hfill}%
   settowidthlabelwidth{makelabel{#1}}%
   setlengthleftmargin{labelwidth}
   addtolengthleftmargin{labelsep}}}
  {end{list}}

documentclass{article}
usepackage{calc}
usepackage{enumitem}

begin{document}

begin{description}[leftmargin=!,labelwidth=widthof{bfseries The longest label}]
  item[The longest label] text
  item[Short]  long long long long long long long long long long long long long long long long long long long long long long long long long long long long long long text
end{description}

end{document}

documentclass[twocolumn]{article}
usepackage{scrextend}
begin{document}
begin{labeling}{alligator}  % alligator is the longest label
    item [textbf{ant}]really busy all the time
    item [chimp] likes bananas
    item [alligator] very dangerous animal, sharp teeth, long
     muscular tail and a bit of text that is longer than one
     line and shows the alignment of text quite nicely
end{labeling}
end{document}