Two signals \( s_1(t) \) and \( s_2(t) \) are considered orthogonal over an interval [0, T] if their product integrated over that interval is equal to zero, which is mathematically represented as \( \int_{0}^{T} s_1(t)s_2(t) dt = 0 \). This is a mathematical property for signals which is essential in many digital communication systems.