How do scientists define temperature?

Speed: The molecules of air and water bounce and hit each other at certain rates. Higher the temperature, faster collisions. Lower the temperature, the slower the molecules collide.

Answer

Temperature is actually a measurement of energy. Every molecule has mass and velocity, the energy of which is E=¹/₂mv². To get picky it is the root mean square velocity as any population of molecules has a normal distribution of velocities. The units of temperature could then be thought of as joules per molar volume.

Answer

To simplify, temperature is the average speed of particles. How fast particles are moving depends on their kinetic energy. Though the definition would state it measures speed, it is actually the underlying amount of energy particles possess. But in any case, speed is a direct effect of this energy; so they can actually be synonymous in this case.