1 word

Incremental
adj : increasing gradually by regular degrees or additions; "lecturers enjoy...steady incremental growth in salary"
in·cre·ment
n.

1. The process of increasing in number, size, quantity, or extent.
2. Something added or gained: a force swelled by increments from allied armies.
3. A slight, often barely perceptible augmentation.
4. One of a series of regular additions or contributions: accumulating a fund by increments.
5. Mathematics. A small positive or negative change in the value of a variable.