What physical form does lightning belong to? Is it a gas or a solid?

Lightning is a natural phenomenon, not an object, so it can't be a gas or a solid.

When the weather is hot and humid, the water on the ground is heated and turned into steam, which rises with the heated air on the ground and meets the cold air in the air, so that the rising water vapor condenses into small water droplets and forms cumulus clouds. The water droplets in the cloud are blown up by the powerful airflow and split into some small water droplets and large water droplets. Larger water droplets are positively charged and smaller water droplets are negatively charged. Small water droplets gather with the wind to form a negatively charged Lei Yun; Larger water droplets with positive charges often fall to the ground to form rain or suspend in the air. Due to electrostatic induction, negatively charged Lei Yun induces a positive charge on the earth's surface. In this way, a large capacitance is formed between Lei Yun and the earth. When the electric field intensity is very strong and exceeds the breakdown intensity of the atmosphere, a discharge occurs between Lei Yun and the earth, which is generally called lightning.

In this process, it mainly produces: thunder is acoustic energy and lightning is electrical energy. It also produces light energy.

Supplement: Electricity is generated by the directional motion of electric charge. Charge needs a carrier, which means that the charged body is a substance, but electricity itself is a concept, not a substance.