The Weibel instability is a plasma instability present in homogeneous or nearly homogeneous electromagnetic plasmas

Plasma (physics)

In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 which possess an anisotropy in momentum (velocity) space. In other words, all the electrons are moving in one direction. In the linear limit the instability causes exponential growth of electromagnetic fields in the plasma which help restore momentum space isotropy. In other words, when this effect is pronounced, it creates electrostatic waves which fight the electrons from all moving in one direction.