Team work.
I assume you are referring to the use of the term in relationship to goals and projects and not the original description of a stone marking off miles on a road. While it is not actually an antonym for a milestone, the opposite could be considered to be "setback".
Meeting at the Milestone was created in 1947.
Milestone S.r.l. was created in 1996.
Milestone Records was created in 1966.
Milestone Media was created in 1993.
Milestone Media ended in 1996.
Zero Milestone was created in 1923.
Milestone Systems was created in 1998.
Milestone Systems's population is 2,011.
Milestone Systems's population is 375.
Download Milestone 1.9 (Super Tux 0.3.3): If you have a Windows OS:
A milestone is a significant step in the project life span. In general expect to see eight milestones per project as a broad rule of thumb. Most projects that recognize milestones will last more than a month, and probably less than five years. Each milestone will have at least one measurable, quantifiable, recognizable completion associated with it.