Neural Network Optimization steps order
One iteration means that you do one forward pass and one backward pass for a single batch containing batch_size examples. Then you move on to the next batch. Also see this post on SE SO.
Note that this is not identical to an epoch. An epoch is only completed when all examples of your dataset have been passed through your network. And as long as your batch size is less than the number of datapoints you will need $\frac{n}{\text{batch size}}$ (with $n$ being the sample size) iterations to complete an epoch. Also see this answer on SE DS.