reaction coordinate
Think of the elevation profile of a hiking trail — that side-on chart showing how high you are at each point along the walk, with its climbs and dips. The horizontal axis is not your map position but simply how far along the trail you have travelled. The reaction coordinate is the chemistry equivalent: a single line that measures how far a reaction has progressed from start to finish.
More precisely, the reaction coordinate is a one-dimensional measure of the progress of a reaction along its lowest-energy path, condensing all the complicated atomic motions — bonds stretching, angles bending, atoms approaching and leaving — into one number that runs from reactants on the left to products on the right. Plotting potential energy against this coordinate gives the reaction profile or energy diagram, whose peaks are transition states and whose dips are intermediates or the final products.
Why it matters: this diagram is one of the most useful pictures in chemistry, instantly showing the activation energy (the height of the climb), whether a reaction releases or absorbs energy (whether products sit lower or higher than reactants), and how many steps a mechanism has (how many peaks). The honest caveat is that the reaction coordinate is an abstraction: a real reaction wanders through a many-dimensional energy landscape, and squeezing it into one axis is a simplification, useful for intuition but not the full story.
Draw a curve that starts at the reactants' energy, rises to a single hump, and comes down to a lower level for the products. The horizontal distance is the reaction coordinate; the hump is the transition state; the height of the hump above the start is the activation energy; the drop from start to finish tells you the reaction gives out energy.
An energy-versus-progress curve: peaks are transition states, dips are intermediates.
The horizontal axis is progress of reaction, not time and not distance in space. A two-peak profile means a two-step mechanism with an intermediate sitting in the valley between.