Since it is still permissible to do melacha (work forbidden on shabbat) including lighting shabbat candles until shki'a (sunset), which is 18 minutes (or 40 minutes in Jerusalem) following "candle-lighting time", why do we still call it by that name?
Dr. Yehudah (Leo) Levi, in his Jewish Chrononomy (Hebrew section, p. מ, third footnote) cites two reasons that have been suggested for the 18-minute standard:
It is in deference to the opinion of Sefer Yereim that twilight (bein hashemashos) begins a 3/4-mil walk before sunset (not after it, as in other opinions). Opinions on the length of time it takes to walk a mil vary, but the largest figure is 24 minutes, so 3/4 mil would be 18 minutes. (Candle-lighting would be one of the last things done before Shabbos starts, for the practical reason that this way the light will last longer into the night; indeed, in the Mishnah (Shabbos 34a) it's the last item on the list of things to tell one's household to do before Shabbos.)
The Gemara (ibid. 35b) states that after the blast of the shofar signaling that it's time to light Shabbos candles, the signalman would wait "the time it takes to roast a small fish" and then blow the final three blasts indicating that Shabbos has begun. We don't know how long that period is, but we find in halachah that roasting takes the time it takes to walk a mil (Maggid Mishneh to Hil. Maachalos Asuros 6:10), for which the smallest figure given in halachah is 18 minutes.
If someone couldn't light earlier, then we go with the standard halachic understanding that the deadline is sunset. But either of these reasons explains why it's best to light candles at least 18 minutes earlier (and there are some communities where the standard time is even earlier than that - in Jerusalem, for example, candle-lighting time is 40 minutes before sunset).
The "candle-lighting" in the term (English translation of הדלקת נרות or ליכט בענטשען) probably refers to the ritual act of lighting candles which is an obligatory act (first recorded by our sages) performed on each Friday evening, and not to the act of lighting candles in general and for other purposes.
The time people generally light candles for Shabas is called candle-lighting time. I don't think that that really requires a source: it's standard use of the English language. That means that there may be different views as to what candle-lighting time (even at the same latitude and longitude): many in America use 18 minutes before sunset, but if a certain synagogue tends to pray very early in the summer and women are instructed not to light before p'lag hamincha, then the synagogue's schedule may list p'lag hamincha as "candle-lighting time". In other words, I don't think the term has any special meaning beyond "time to light candles".
But I have no proof of this.