The singularity usually refers to the idea that although the rate of relative improvement of a technology is approximately constant at a constant (human) intelligence level ([dy/dt]/y ≈ c), producing exponential growth, if said constant were actually proportional to intelligence and the technology under improvement being intelligence itself ([dy/dt]/y ≈ cy), then solving the differential equation leads to a blow-up to infinity in finite time, a mathematical singularity.

If instead additional intelligence does little to speed up AI development (due to the need for other inputs like caputal and time), you could get a world where AI becomes better than humans at AI development and begins a cycle of recursive self-improvement without explosive growth leading to a singularity.