When we say STEM, do we simply mean any of the four subjects or do we mean those areas in which some of the four, ideally all four, overlap? My sense is that most people simply mean Science OR Technology OR Engineering OR Mathematics when referring to STEM, though there are some efforts under way, including some at the National Academies, that mean to explore whether education can benefit, just as research already does, when the four are somehow linked.
Now add the arts and you get STEAM. And since no one in his or her right mind would simply want to add "arts", believing that it belongs in the same space (for then, why not add history, philosophy etc. and end up including everything and anything), there is a specific theory of action that those who talk about STEAM have in mind when adding arts to science, technology, engineering and math. So, why or how does the A then fit into the STEM to form STEAM?
I see two major claims. The first one refers to art as a different way of perceiving and knowing and dealing with the world, as a means to expand the toolbox of science and engineering. In engineering, which some summarize as design under constraints, art can provide a useful tool to make the engineered world or object more appealing and thus acceptable and useful to people. In science it is seen as a different way of seeing the world, a heuristic that leads to a better, or at least different understanding of the world. One example for this perspective is visualization, which empowers science research just as it does science education (see the Gordon Research Conference on Visualization in Science and Education).
The second claim is based on the limitations of scientific research and of engineering design, which some see as lacking creativity and fun. Art, in this view, is a means to free the scientist's and engineer's mind. It should be noted that highly selective STEM specialty schools encourage their students to pursue the arts, be that poetry, music, theatre, or any other aspect of it.”
- More here and More on Seed