a finger: a digit of the hand (and by extension a toe, or a finger's width as a small measure)
The plain picture is a single digit of the hand. Its most frequent job in the Bible is a working one: in the sacrifices of Leviticus the priest dips his finger in blood or oil and dabs it on the altar or on a person. The same finger points, plays, and shapes: it is put out to accuse, it is set to fight on a harp string, and it is the finger that carves and casts the idols people then bow down to.
Because it simply means a digit, the word also stretches naturally in two directions. It covers the toe, the digit of the foot, in the two accounts of the man born with twenty-four digits, six on each hand and each foot. And a finger's width becomes a small unit of measurement, as when a bronze pillar is said to be four fingers thick.
Its most memorable use is figurative: the finger of God. When the stone tablets are written by the finger of God, when the sky is called the work of his fingers, and when Egypt's magicians admit that a plague they cannot match is the finger of God, the phrase is not shrinking God down to a body part. It is a vivid way of saying that God has acted directly and personally, by his own power, close up and unmistakable.