I'm an engineer and the terms can mean the same thing, but not always. There are small differences. 'Meter' will be used for a device that measures the rate at which some resource is used. A flow meter for water would tell me at what rate the water is flowing through some device. It might also tell me the total amount that has been used or, if I monitor the flow rate, I can take the sample rate and multiply it by time. This concept cannot be applied to a gauge, indicator or transmitter. A gauge measures some value at a particular moment. A meter could be used here. The difference is that the gauge will not tell you the rate of usage. A pressure gauge gives you the fluid or air pressure at any given time, but it won't tell you how much or at what rate the fluid or air is being used. An indicator usually employs some sort of arrow or light to tell you the current state of some measured value. It is used when there is simply an on/off state for the device. For example, if I have a red light that lights when the water tank level is too high, I would call it an indicator (or warning) light. I don't know exactly what the level is - I just know that it's too high. I would not use the other three terms for this function. If it tells me the current value, then I could substitute gauge or meter. A transmitter sends some sort of information from one place to some receiver, usually not through direct wiring, but by radio or light transmission. It may be some measurable value or it may just indicate the state of a device or the identity of some component. If I put a pressure sensor on a saddle that sends a signal when there is steel placed there, I would call it a transmitter. I emphasize that at times, these four words are interchangeable - sometimes not. Hope this helps.