All the data is taken from the chart in the blog on "Why the profit is maximised when the marginal cost curve crosses the marginal revenue curve"
dTC : Change in total cost
VC : Variable cost
ATC : Average total cost; AC is the average cost
dATC : Change in ATC
AVC : Average variable cost; VC is the variable cost
dAVC : Change in AVC
FC : Fixed Cost.
Q : Quantity.
dQ : Change in Quantity.
MC : Marginal cost.
TC = VC + FC
dTC = dVC+dFC
Similarly, the marginal cost is the next new comer. Marginal cost is the next change in the cost (per unit). This next change, if it is more than the average, then the average will increase or vice versa.
Note that MC is the new comer for both the the averages as we proved earlier.