# What is the definition of a component in a phase diagram?

I have been trying to understand the concept of a component in relation to phase diagrams in my materials class. I have tried searching online and found that the definition is supposed to be that a component is a singular element, however I have also found a water (a 2 element compound) phase diagram is listed as a singular component phase diagram, which contradicts the previous definition of a component.

So is a component all relative to the temperatures/concentrations it operates under? Like, if a liquid is made of two elements and you're looking at it between 20-100 degrees Celsius and it only separates into its individual elements after 100 degrees Celsius, then would it be classified as a singular component phase diagram under 100 degrees Celsius?

So in short I need help understanding what a component is in relation to phase diagrams.

Here one has pure GdF$_{3}$ on the left, and pure CsF on the right. The composition axis is no longer in atomic fraction (my bad on labeling the axis), but is instead a weighted average of the pure compounds. So, for instance, the purple line representing Cs$_{3}$GdF$_{6}$, is found at a 'composition' of 0.75. This is because it is an amalgamation of 3 CsF units and 1 GdF$_{3}$ unit, so it is (3/4) units towards CsF. Similarly, the CsGd$_{2}$F$_{7}$ is one CsF and two GdF$_{3}$ units, so is (1/3) units towards pure CsF.