Complex sentence: In order to build the first level of the "k"-d tree, the median element of the first index array (array 0) is chosen; let the "value" of this median element be called median 0.
Split into simpler sentences: In order to build the first level of the "k"-d tree, the median element of the first index array (array 0) is chosen. Let the "value" of this median element be called median 0.