The two types I see a lot are 1+1 or 1:1.
The former is clear, and refers to stock plus the same quantity of water.
The second can cause confusion - does it mean 1 part stock in 1 part working solution (i.e. just stock) or does it mean the same as 1+1 (working solution is 1 part stock plus 1 part water).
Another way of designating undiluted stock would be 1+0.
The undiluted developer will require more (correction: less) time than the diluted developer.