The various terms for voltage
As suggested by other answers, most of this is pretty arbitrary anyway


Vpp : peak-to-peak voltage (for AC waveforms), historically this would also be used for the programming voltage for EEPROM or flash memory (particularly those devices that did not generate their own programming voltage on-chip).
Vp : peak voltage (again, for AC) referenced to system ground, or 0V.  May also be used for pull-up voltages (which could also be Vpu).
Vcc : positive power supply for many IC's, traditionally this referred to BJT based ICs, the 'cc' referring to the collectors of the integrated transistors.  Often this was matched with a negative supply, Vee ('ee' referring to the emitters of the transistors).
Vc : Collector voltage for a BJT, similarly Ve, Vb may also be used for the emitter and base while Vs, Vd and Vg may be used for the source, drain and gate of FETs.
Vr : reverse voltage, particularly when referenced to diodes.  You may also encounter Vz used to indicate a zener voltage.  Vf is used to indicate the forward voltage drop of the diode.
Vdd : positive power supply for many IC's, traditionally this referred to FET (NMOS, PMOS, CMOS) based ICs, the 'dd' referring to the drains of the integrated FETs.  Often this was matched with a negative supply, Vss ('ss' referring to the sources of the FETs).
Vi : input voltage.
Va : used to indicate an internal analogue voltage point.
Vt : May be used for the Thevenin equivalent voltage, or as suggested by WhatRoughBeast the threshold voltage (for a comparator or similar for example), or the termination voltage (also known as Vtt in the case of DDR=type memories).
Vo : Output voltage for op-amps and the like.
0V : Zero-volts, not to be confused with Vo, refers to the system ground.
Also,
Vhsys : may be used for the hysteresis voltage of a comparator type circuit.