Continuous aerobic exercise held at one intensity for the whole session, as opposed to intervals that alternate hard and easy.
A 45-minute run at one pace, an hour on the bike, a long row. Most steady-state work is done in zone 2, but the term describes the structure rather than the intensity.
It is undervalued by clients who have been told intervals are more efficient. Intervals produce similar aerobic adaptations in less time but cost far more recovery, which matters for anyone also lifting three or four days a week. Steady state is what can be added without taking anything away.
The main practical advantage is compliance. Clients will do a 40-minute walk or cycle at an easy pace repeatedly, week after week, in a way they will not do with hard intervals.