Closed loop/Open loop
Fuel injection systems are always trying to deliver just the right amount of fuel to the engine to achieve the best combustion, which comes from a stoichimetric air/fuel ratio. To do that, oxygen sensors in the exhaust constantly monitor the oxygen content of the combusted gasses or Exhaust Gas Oxygen (EGO). Too much oxygen means that not enough fuel is available, while too little oxygen means too much fuel. This constant check and balance (or feedback loop) of fuel injected and EGO is called closed loop operation. Most cars operate in closed loop at part throttle and low loads. At WOT, the ECU jumps out of closed loop operation to open loop, which just injects a predetermined amount of fuel for a typically rich air/fuel mixture. Newer cars, or stand alone ECUs, can potentially use Closed Loop management even at wide open throttle since they can use wide band oxygen sensors to target A/F mixtures, even at WOT, that older narrow band sensors can't measure.
Equivalence ratio
Instead of using air/fuel ratios (A/F), which are a ratio of the mass of air divided by the mass of fuel consumed by an engine, engineers like to take it a step further. By dividing the actual A/F ratio by the ideal stoichimetric A/F ratio, 14.7:1, you get the fuel/air equivalence ratio, signified by the greek symbol (phi). So now we know if a mixture is rich, is greater than 1.0. Lean is less than 1.0. Why all the trouble? Because engine nerds don't always deal with gasoline like we do. Other fuels mixes, like diesel, hydrogen, ethanol or even ethanol gasoline, have different stoichimetric A/F ratios than the 14.7:1 we're use to for gasoline. In fact, common California unleaded pump gas, which has 10-percent ethanol, or E-10, has a stoichimetric A/F ratio as low as 14.1:1. We typically don't mention this since this is well within the compensation ranges of most ECU closed loop operations, but it is relevant. Sometimes we like to confuse ourselves and also use a relative air/fuel ratio called (lambda), which is the inverse of , where lean is greater than 1.0 and rich is less than 1.0. That's why people often call wide band oxygen sensors lambda gauges.
Stochiometetry
We use this term a lot since there's no better word for it. Stoichiometery simply means the complete or balanced chemical reaction of two or more properly proportioned reactants into another form without any leftovers. Burning fuel abides by the same chemical laws, and if you work out the molar balance, conventional gasoline and all the components of air react completely with a mass ratio of 14.7 parts air to one part gasoline. Stoichimetric combustion isn't just some geek number. It's also the ideal ratio for the most efficient fuel consumption, power production, emissions reduction, and highest combustion temperatures.