Static electricity is an imbalance of electric charges within or on the surface of a material. The charge remains until it can move away as an electric current or by electrical discharge. The word "static" is used to differentiate it from current electricity, where an electric charge flows through an electrical conductor.

A static electric charge can be created whenever two surfaces contact and/or slide against each other and then separate. The effects of static electricity are familiar to most people because they can feel, hear, and even see sparks if the excess charge is neutralized when brought close to an electrical conductor (for example, a path to ground), or a region with an excess charge of the opposite polarity (positive or negative). The familiar phenomenon of a static shock – more specifically, an electrostatic discharge – is caused by the neutralization of a charge.

Materials are made of atoms that are normally electrically neutral because they contain equal numbers of positive charges (protons in their nuclei) and negative charges (electrons in "shells" surrounding the nucleus). The phenomenon of static electricity requires a separation of positive and negative charges. When two materials are in contact, electrons may move from one material to the other, which leaves an excess of positive charge on one material, and an equal negative charge on the other. When the materials are separated, they retain this charge imbalance. It is also possible for ions to be transferred.

Electrons or ions can be exchanged between materials on contact or when they slide against each other, which is known as the triboelectric effect and results in one material becoming positively charged and the other negatively charged. The triboelectric effect is the main cause of static electricity as observed in everyday life, and in common high-school science demonstrations involving rubbing different materials together (e.g., fur against an acrylic rod). Contact-induced charge separation causes one's hair to stand up and causes "static cling" (for example, a balloon rubbed against the hair becomes negatively charged; when near a wall, the charged balloon is attracted to positively charged particles in the wall, and can "cling" to it, suspended against gravity).

Applied mechanical stress generates an electric polarization and in turn this can lead to separation of charge in many types of materials. The free carriers at the surface of a material compensate for the polarization induced by the strains.

Heating can generate electric polarization, which in turn can lead a separation of charge in certain materials. All pyroelectric materials are also piezoelectric and do not have inversion symmetry.

A charged object brought close to an electrically neutral conductive object causes a separation of charge within the neutral object. This is called electrostatic induction. Charges of the same polarity are repelled and move to the side of the object away from the external charge, and charges of the opposite polarity are attracted and move to the side facing the charge. As the force due to the interaction of electric charges falls off rapidly with increasing distance, the effect of the closer (opposite polarity) charges is greater and the two objects feel a force of attraction. Careful grounding of part of an object can permanently add or remove electrons, leaving the object with a global, permanent charge.

Removing or preventing a buildup of static charge can be as simple as opening a window or using a humidifier, to increase the moisture content of the air, making the atmosphere more conductive. Air ionizers can perform the same task.