Plasma is an electrically neutral gas in which all positive and negative charges--from neutral atoms, negatively charged electrons and positively charged ions--add up to zero. Plasma exists everywhere in nature (for example, lightning and fluorescent light bulbs), and it is designated as the fourth state of matter (the others are solid, liquid and gas). It has some of the properties of a gas but is affected by electric and magnetic fields and is a good conductor of electricity. Plasma is the building block for all types of electric propulsion, where electric and/or magnetic fields are used to accelerate the electrically charged ions and electrons to provide thrust. In ion thrusters, plasma is made up of positive ions and an equal amount of electrons.