Sentence:  Since the line joining the circumcenter and orthocenter of a triangle is its Euler <WSD>line</WSD>, we see that this parabola is the envelope of the <WSD>Euler lines</WSD> of the triangles "A<sub>i</sub>". 
Meaning of the word marked with <WSD> tags:  (geometry) A line that, for a given triangle, passes through several important points, including the circumcentre, orthocentre and centroid; an analogous line for certain other 2- and 3-dimensional geometric figures.