Tangent Line: A straight line that touches a curve at a single point without crossing it, representing the instantaneous rate of change (slope) at that point.
Derivative: A measure of how a function's output value changes as its input value changes, formally defined as $f'(x) = \lim_{h \to 0} \frac{f(x+h) - f(x)}{h}$.
Differential: An infinitesimal change in a function's value resulting from an infinitesimal change in its input, often denoted as $dy = f'(x)dx$.