Plan Position Indicator. A display that represents the position of an object by bearing and range, (which way and how far). You know, like those radar screens with the line that sweeps around and around and the little blips that show where the bogies are.

If it is PPi (with the lower case i), it's inorganic pyrophosphate, which is PO4-, or what get's released when ATP gets converted to ADP, or ADP to AMP.

It's inorganic in that there is no longer any carbon or hydrogen involved with it.

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