phase
The phase tells you where in its cycle an oscillation is at this moment: just starting, passing through the middle, or paused at the top. Picture two identical swings both moving; one is at its highest point while the other is at the bottom. We say they are out of phase. Phase is the bookkeeping that answers: what stage of the wobble are we at?
Precisely, in x(t) = A cos(omega t + phi), the phase is the entire angle inside the cosine, (omega t + phi). The part phi is the phase constant, or initial phase, fixed by where the object happens to be at time t = 0. Comparing two oscillations, their phase difference tells how they line up: a difference of 0 means they move together (in phase), while a difference of pi radians, or 180 degrees, means they are exactly opposite (anti-phase). Phase is measured as an angle, in radians or degrees.
Phase becomes essential when oscillations meet: whether two waves add up or cancel out depends entirely on their phase difference, and the same idea runs through alternating-current circuits and light interference. A gentle reminder: phase is an angle even when the motion itself is a straight-line bob. That angle comes from the circular-motion picture behind simple harmonic motion.
Two pendulums released from opposite sides at the same instant are pi radians (180 degrees) out of phase: when one is at its far left, the other is at its far right, forever mirror images while their amplitudes last.
Phase difference tells you how two oscillations line up: 0 is in step, pi is exactly opposite.
Phase is an angle (radians or degrees), even for straight-line motion; it comes from the circular-motion picture behind SHM, not from any actual spinning of the object.