Deep work
Deep work is sustained concentration on a demanding task with no switching, and the usual name for the state that cognitively valuable work requires.
The term comes from Cal Newport's 2016 book, which defines deep work as professional activity performed in a state of distraction-free concentration that pushes your cognitive capabilities to their limit. Its counterpart is shallow work: the logistical, non-demanding tasks that can be done while distracted, which expand to fill whatever room they are given.
The mechanism people report is a ramp. Demanding work takes a while to get into, so one hour of it is worth much more than four fifteen-minute pieces, and an interruption costs the ramp rather than the minute it took. That is the argument for protecting a window, as opposed to the argument for finding more hours in total.
It is worth knowing that the term is a popular synthesis rather than a settled research result. The components under it have support, including attention residue and the measured cost of interruption. The packaged prescription is advice. What almost nobody disputes is the negative half: a calendar cut into fragments cannot produce work that needs a long run at it.
Why it matters
Deep work and relationship work lose to whatever has a deadline, and they lose the same way. Neither is urgent on any particular day, and both turn out to be what the year was about.