\setupenumerations
  [before={\blank},
   after={\blank},
   alternative=serried,
   title=yes,
   titleleft={(},
   titleright={)},
   titlestyle=normal,
   titlecommand=\groupedcommand{}{\bf.},
   width=broad]

\defineenumeration
   [definition]
   [text=Définition,
    number=no,
    headcommand=\groupedcommand{}{},
    style=italic]

\starttext
\startdefinition{With a title}
\samplefile{lorem}
\stopdefinition
Without title but no dot after the name of the environment.
\startdefinition
\samplefile{lorem}  
\stopdefinition
\stoptext