Indentation in list environment

Question

How can I specify explicitly both the indentation of the label and the actual item's text from the left margin in a list environment. I would like to achieve the following:

Answer 1

Your specifications are not very clear; the following could be what you need:

\usepackage{enumitem,lipsum} % in your preamble


\begin{description}[leftmargin=2cm,itemindent=0cm,
  labelindent=1cm,labelwidth=\dimexpr1cm-.5em\relax,
  labelsep=!,align=left]
\item[X] \lipsum*[4]
\item[First] \lipsum*[2]
\item[Second] \lipsum*[3]
\end{description}

(As always, lipsum is used only to provide dummy text.)

If you want to be able to set the dimensions at usage time, you can define a personal environment, such as

\newenvironment{xdesc}[2]
  {\begin{description}[leftmargin=#2,
     labelindent=#1,labelwidth=#1,
     labelsep=0pt,align=left,style=multiline]}
  {\end{description}}

to be called as

\begin{xdesc}{1cm}{3cm}
\item[X] \lipsum*[4]
\item[First] \lipsum*[2]
\item[Second and more] \lipsum*[3]
\end{xdesc}

where the first argument is, in your notation, sep1 and the second argument is sep2.

Answer 2

If I might throw in a ConTeXt solution again:

\definedescription [list]
    [
      location=serried,
     headstyle=\hskip1cm\bold, % sep1
      distance=2cm,            % sep2
         width=0cm,
    ]

\starttext

% only for illustration
\blackrule [height=1ex, width=2cm]
\blackrule [height=1ex, width=1cm]

\startlist{First}
    \input knuth
\stoplist

\startlist{Second}
    \input tufte
\stoplist

\stoptext

The result looks like this:

In my example I misused the headstyle to insert the first space. Your contraint 1cm measured from the beginning of the label has the drawback, that all labels wider than 1cm print into the list text (see label of second list).