<p>Yeah, I hear you. Euler is even more problematic than Lebesgue.</p>
<p>The biggest problem is when mathematicians can’t even agree on equivalent definitions for something (e.g. rings a.k.a rings with unity a.k.a associative rings).</p>
<p>Multiple equivalent definitions are usually very interesting, as they link 2 often completely different ways of thinking of something (e.g. Heine-Borel or construction of R by Cauchy sequences and construction of R by Dedekind cuts.)</p>