Biology/Zoology:

* Invertebrate: An animal without a backbone

* Insecticide: A chemical used to kill insects

* Incubation: The period of time during which an egg develops

* Immune system: The body's defense system against disease

* Intertidal zone: The area of the seashore between high and low tides

* Intestine: The part of the digestive system between the stomach and the anus

* Isotope: An atom with the same number of protons but a different number of neutrons

Chemistry:

* Ion: An atom or molecule with a net electric charge

* Isotope: See above

* Inorganic: Not composed of or derived from living matter

* Intermolecular forces: Forces that attract or repel molecules

* Indicator: A substance that changes color in the presence of another substance

* Ideal gas: A theoretical gas that conforms to certain physical laws

Physics:

* Inertia: The tendency of an object to resist changes in motion

* Impulse: A change in momentum

* Intensity: The strength of a wave or other physical quantity

* Interference: The combination of two or more waves to produce a new wave pattern

* Inverse square law: A law stating that the intensity of a physical quantity decreases with the square of the distance from the source

Other:

* Ice: Frozen water

* Illumination: The act of providing light

* Information: Data that is processed, organized, and presented in a meaningful way

* Invasive species: A species that is not native to an area and can cause harm to the environment

* Insulation: Material that slows down the transfer of heat

This is just a small sample of the many science words starting with "i". Let me know if you're looking for words related to a specific scientific field!