ahead of all others: the one that comes before the rest, whether in time, in order, or in rank
At its core this word marks the front of a line. What that line is depends on the sentence. Most often it is a line of time or sequence: the first day, the first to arrive, what someone does first before doing anything else. The same word can mark the front of a line of value or status, so it shades into foremost, leading, greatest, or chief, the one who outranks everyone else. These are not separate words but one word measured along different scales: earlier on a timeline, or higher on a ladder.
The two readings can sit very close together, and a few sayings of Jesus deliberately play on the gap between them. When he says the first will be last and the last first, he means rank, not the order people lined up in. When he says whoever wants to be first must be everyone's servant, he is redefining what greatness is, not who got there earliest.
In Revelation the word is lifted into a title. The risen Christ calls himself the First and the Last, claiming to stand at both ends of all things, before everything began and after everything ends. Here first is not one item in a series at all but a name for the one who has no one before him.