Sound is a vibration in the form of a pressure wave that travels through a medium such as gas, liquid or solid. As defined by the Acoustical Society of America, sound is an "Oscillation in pressure, stress, particle displacement, particle velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillation." 
Simply put, sound is what we hear when a sound wave passes through a medium and reaches our ears. On Earth, sound travels through our atmosphere which consists primarily of nitrogen and oxygen molecules. A sound pressure wave is created when a force causes an object to vibrate. The vibration creates a sound pressure wave that travels through the air away from the sound source. When these waves of vibrating nitrogen and oxygen molecules hit our ears, they vibrate our eardrums as well as the bones inside our inner ears. These vibrations are converted by the cochlea into electrochemical impulses that our brains perceive as sound.
Sound is a form of energy
When a guitar string is plucked, a certain amount of force is transferred to the string. The string is given energy from the motion of plucking, causing the strings to vibrate. This vibration is an interruption of the air pressure around the string caused by the oscillation of kinetic and potential energy the string has. The interruption of the air pressure creates a sound pressure wave.
Guitar strings, like the strings on many other instruments, are strung with varying levels of tension. This tension gives the string potential energy. When the string is plucked, a certain amount of the potential energy is converted to kinetic energy. As the string vibrates, it is oscillating back and forth between the transfer of kinetic energy and potential energy. This is what gives the string resonance and a sustained sound. As the kinetic energy is being transferred back and forth, this vibration is causing waves of pressure in the air. Those vibration waves spread outwards in all directions in the form of a sound energy wave. These sound waves then begin bouncing off the objects in the room, the walls, the floor, the ceiling, all before entering your ear.
Vibration is key. A trumpet sound is made by vibrating lips on a mouthpiece, a woodwind sound is made by a vibrating reed, a drum sound is made by a vibrating drum head, and our voices are made by vibrating vocal chords in your throat. Vibration is the underlying principle to all sounds.
^ ANSI/ASA S1.1-2013 (Link to: https://asastandards.org/Terms/sound-2/)