Continuity:A function is continuous at point $c$ if $\lim_{x ->c} f(x) = f(c)$ and it is defined at that point.
Indeterminate Forms: Expressions like $\frac{0}{0}$ or $\infty - \infty$ which do not have well-defined values and require special techniques to evaluate.
$\epsilon-\delta$ Definition: $\epsilon-\delta$ definition formalizes the concept of a limit, stating that for every $\epsilon >0$, there exists a $\delta >0$ such that if $0<| x-c |<\delta$, then $|f ( x ) -L |<\epsilon$.