There is simple definition for candles which is an ignitable wick embedded in candle wax and/or other flammable solid substance.

For a candle to burn, a naked flame is necessary to light the candle’s wick.

Once the heat vaporizes the candle fuel, fuel combines with oxygen and forms flame.

Then flame provides sufficient heat to keep the candle burning. As this process continues candle becomes shorter till burning candle fuel runs out.