It took him a few weeks to make the adjustment after starting his new job.

adjustment 是动词 adjust 的名词形式，由 adjust 加上名词后缀 -ment 构成。adjust 的意思是"调整、适应"，所以 adjustment 指的是"调整的过程或结果"，在这里特指"适应新工作的过程"。