Engulfing candle - precise definition

The candle on the right is often classed as an engulfing candle simply because the body engulfs the body of the candle before it. However, in this case the period before it was the start of the Asian session and very quiet. Therefore, surely it should have to engulf the candle prior to this otherwise it is no different to the other candle over to the left of the shot?

Not sure what TF you're looking at but I don't entirely agree. First off, engulfing candles are engulfing candles, whether or not you think they're valid entry signals or not. Re your discretion about validity, if the 2nd candle is the first break out of Asia into London, it's certainly more significant than one inside Asia. But all told an engulfing candle by itself has virtually no statistical validity unless you combine it with something concrete.

Asian session except for yen based pairs do range through a portion of the previous day's action more than trend. Interesting fact for you - the typical retail trader has a higher than 50% win rate, compared to abysmal win rates of London/NY. It says a lot for retail trading styles.

An engulfing candle is defined in relevance to the candle before it so in your example, the right candle is an engulfing candle by definition. Whether you want take a position there or not is a different matter. Reversal candles are more meaningful after trends and at significant price levels (e.g. support/ resistance). On the other hand, the candles formation to the left of the chart looks like a Harami to me.

I agree with both the posts above. What you're talking about really relates to the significance of specific instances of engulfing candles, not to whether they're engulfing (a question which is, in itself, objective and factual).