Electrostatic discharge (ESD) is a sudden and momentary flow of electric current between two differently-charged objects when brought close together or when the dielectric between them breaks down, often creating a visible spark associated with the static electricity between the objects.
A portion of a static discharger on an aircraft. Note the two sharp 3/8" metal micropoints and the protective yellow plastic.
ESD Jacket
A network card inside an antistatic bag, a bag made of a partially conductive plastic that acts as a Faraday cage, shielding the card from ESD.
Electric discharge showing the ribbon-like plasma filaments from multiple discharges from a Tesla coil.
An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an ionized, electrically conductive channel through a normally-insulating medium, often air or other gases or gas mixtures. Michael Faraday described this phenomenon as "the beautiful flash of light attending the discharge of common electricity".
A spark on a spark plug
Lightning is a natural example of an electric spark.
Benjamin Franklin drawing an electric spark to his finger from a key suspended from a kite string.
Gas stove burner - the electric spark flame igniter is shown at the left.
