Deep Work
Focused, distraction-free work on a cognitively demanding task, held for an extended, uninterrupted stretch of time.
Explanation
Deep work (a term popularized by Cal Newport) is professional activity performed in a state of distraction-free concentration that pushes your cognitive abilities to their limit. It's the opposite of "shallow work", email, meetings, small logistical tasks, which feels busy but produces little lasting value.
Example
Blocking a 90-minute window with notifications off to write the hardest section of a report is deep work; answering Slack messages in between small tasks all afternoon is not.