Is there a difference between coordinate system, spatial reference, and projection?

These three words are often seen in various books and documents. projection is the most used, coordinate system is the second, and spatialreference is the last. Recently, I feel more and more uncomfortable. If the three expressions share the same meaning, why should we get three more? I am an easy-to-use person, so this makes me very uncomfortable, tangled for a long time. In my mind, I want the three representatives to have the same meaning, so that I don't have to distinguish them any more, because I have never been able to understand them before.

Today, we have seen this mixed use in the ArcGIS development help documentation. ESRI is the industry leader. If people say this, it means the same for the time being. However, the psychological aspects are not completely accepted. After all, the three are completely different from each other.

 

Http://help.arcgis.com/en/sdk/10.0/arcobjects_net/componenthelp/index.html#//0049000000n6000000

/// <Summary>

///Generate a spatialreference by setting it's default projection.

/// </Summary>