Answer:
A "Relaxation Oscillator" is an "R/C" oscillator, in which a Capacitor is charged from a DC (Direct Current) fixed voltage source, through a resistor.
Connected across this Capacitor is a "device" that, initially, does NOT conduct any current. But when the Voltage on the Capacitor reaches a certain value (depending upon the characteristics of the "device") the "device" will "break down" or "avalanche", or suddenly drastically decrease in resistance.
This causes the "device" to quickly start to discharge the Capacitor. However, when the Voltage across the Capacitor and its parallel connected "device' decreases with the discharge cycle, the "device" will suddenly return to its "non-conducting" state, and the Capacitor again begins to re-charge.
The Capacitor again reaches the "break down" voltage of the "device", and the process continues, thus effectively creating "oscillations".
One version of this circuit uses a simple Neon lamp (such as a NE-1) as the "device". Typically, the "break down" or "ignition" voltage of a neon lamp is around 70 volts. At this point, the gas in the lamp "ionizes", becoming highly conductive.
However, when this voltage drops (as the lamp discharges the Capacitor) to about 50-60 volts, the lamp "goes out" (the neon gas is no longer ionized or conducting), and the Capacitor again starts to charge through the resistor.
This form of the "Relaxation Oscillator" will not operate at high frequencies (such as Radio Frequencies), and was often used to just generate a "flashing light" (the Neon lamp).
With the advent of semiconductors, a device called the Unijunction Transistor (UJT) was developed. It contained three leads, the Emitter, Base 1, and Base 2.
The junction of the Resistor and Capacitor is connected to the Emitter, with the "bottom" of the Capacitor (and the negative side of the Voltage Source) connected to Base 1 Base 2 is connected to the positive side of the Voltage Source.
Again, when the voltage on the Capacitor reaches the "break down" level, the UJT's "resistance" between the Emitter and Base 1 drastically drops, discharging the capacitor. However, when the discharging current through the UJT drops below a certain level, it again develops a high resistance between its Base 1 and the Emitter, and re-charging of the Capacitor begins again.
UJT circuits can operate at higher frequencies than the Neon bulb circuit. Often a resistor is inserted between Base1 and the common or negative side of the voltage source, and the resulting discharge current produced voltage across this resistor is used to trigger other devices. Also, the voltage waveform across the Capacitor approaches a linear "sawtooth" waveform (depending on circuit component values), but not nearly as linear as other approaches.
"Devices" of this type are considered to have a "negative resistance" characteristic, which is another discussion in itself.