A wave is described as a disturbance that travels through a medium. A wave displaces energy from one point to another. The shape that moves along the medium (with a single input of energy) is a wave pulse.
The diagram shows a continuous wave, not a pulse (meaning a continuous input of energy).
The wave has equally spaced crests and troughs, its shape is called a sine wave.
The distance between any point and the next identical point is
called the wavelength, its symbol is $\lambda$.
Waves can be studied using a ripple tank. An electric motor is used to vibrate a straight bar or a small dipper which is touching the sureface of water in a tank. The vibrations of the small dipper create a pattern of concentric circles
It produces the pattern in the diagram. The blue circles mark the crests of the waves produced.
Each blue circle is a wavefront. A wavefront is a line joining points at the same position along a wave. The red circles are troughs and are also wavefronts.
The waves move at right angles to the wavefronts, as shown by the green arrows in the diagram.
The time it takes an oscillation or a wave to repeat is called the periodic time or the period. Its symbol is T.
The exact repeat of a wave or oscillation is one cycle.