piling up; accumulation; stacking
Refers to the act or result of stacking things up, often used literally for physical objects or figuratively for achievements, data, or experience.
There is a pile of documents on the desk.
The accumulation of small successes leads to big results.
Stacking up the cardboard boxes turned out to be harder than expected.
Success is the result of an accumulation of small efforts.
蓄積 is a more formal, often abstract term for accumulation (e.g. of data or fatigue), whereas 積み上げ keeps a stronger sense of physically or gradually building something up.
gradualist approach; build-up approach
Compound of 積み (noun form of verb 積む, 'to pile up') and 上げ (noun form of verb 上げる, 'to raise'). The combination literally means 'piling and raising', i.e., stacking things up.