How are these two definitions equivalent?

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Definition in the textbook enter image description here

definition-edge cut is the set of edges which disconnect the graph if we remove it from the edge set of the graph.

How are these two definitions equivalent?

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The first definition defines that if you can divide your graph into two disjoint sets of vertexes. And the edge-cut as the set of edges that connect these two sets of vertexes.

The second definition defines and edge cut as a set of edges that would disconnect these two sets of vertexes.

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They are not completely equivalent -- for example, consider

           5
          / \
 1---2---3---4

The set $\{23\}$ is an edge cut according to both definitions (take $S=\{1,2\}$, for example).

The second definition appears to allow $\{23,34\}$ to be an edge cut: removing those edges certainly disconnects the graph. I don't think it is standard to consider $\{23,34\}$ a cut, however -- the definition should be that it is a minimal set of edges whose removal will disconnect the graph.