1 [ uncountable ] the act of making sth better; the process of sth becoming better
2 [ countable ] a change in sth that makes it better; sth that is better than it was before
Oxford Advanced Learner’s Dictionary, 8th edition
而progress的含義是
1 the process of improving or developing, or of getting nearer to achieving or completing sth
2 movement forwards or towards a place
從以上可知improvement的基本意思是從不完善到更完善的過程或變化，而progress是指與要實現的目標或要達到的目的地比以前更接近了
以上單詞英語定義引自英語牛津詞典