Existence and Uniqueness Theorem -> A mathematical theorem that can prove that a solution to a given problem is both possible (existent) and / or there is only one, zero, or multiple solutions (uniqueness). Used mostly in differential equations
Heaviside transform - "Incredibly useful in solving an ordinary differential equation with homogeneous coefficients, this transform involves guessing a general solution of an exponential raised to the negative [kt] power." - [2]
Matchstick construction -> "It states that every point that can be constructed with a straightedge and compass, and no other points, can be constructed using identical movable line segments. The nickname for the movable line segments gives this construction its name."
Sum of the nonconservative forces -> "In a general one dimensional system, it is the difference between the time derivative of the derivative of the lagrangian with respect to velocity and the derivative of the lagrangian with respect to position (that is, d/dt(dL/dq)-dL/dq)"