What does the "d" stand for in this question

Sn = (1/2)n[2a1 + (n - 1)d]

The common difference between each number in a sequence or series

d is common difference in 2 adjacent terms.
e.g. even numbers 2, 4, 6, ..
common difference is 4 – 2 = 2, 6 – 4 = 2
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