Minimal Spanning Tree vs Minimum Spanning Tree
A minimum weight edge is an edge whose weight is minimal among all edges. There can be more than one minimum weight edge. All minimum weight edges have the same weight.
Similarly, a minimum spanning tree is a spanning tree whose weight is minimal among all spanning trees. There can be more than one minimum spanning tree. All minimum spanning trees have the same weight.
In some cases, there is a distinction between minimum and minimal. For example, a minimum vertex cover is a vertex cover whose weight is minimal among all vertex covers. In contrast, a minimal vertex cover is a vertex cover, any proper subset of which isn't a vertex cover. Finding a minimal vertex cover is easy – start with the set of all vertices, and repeatedly remove a vertex as long as it is possible to do so while maintaining the property of covering all edges. In contrast, finding a minimum vertex cover is expected to be hard (the corresponding decision problem is NP-hard).
The same property of being minimal doesn't make much sense for edges and spanning trees. When people say minimal edge or minimal spanning tree, they probably mean minimum (weight) edge and minimum spanning tree.