Her improvement in grades this semester has been very impressive.

impressive 是形容词，由动词 impress（给...留下深刻印象）加上后缀 -ive（表示"具有...性质的"）构成，意思是"给人深刻印象的、令人钦佩的"。 在句中形容进步的程度之大，给人留下深刻印象。