Momentum is defined mathematically as p = mv, where p represents momentum, m is mass, and v is velocity. The discussion highlights the need to consider relativistic effects, leading to the equation p = mv/√(1 - (v/c)²) or p = mvγ, where γ is the Lorentz factor. This factor accounts for the effects of traveling at speeds close to the speed of light, c. The conversation clarifies that the standard momentum equation is insufficient for high-velocity scenarios. Understanding these equations is crucial for accurately describing momentum in relativistic physics.