The phrase "marginal improvement" means a small or slight improvement in something; it refers to making a small positive change or progress. Full definition
It offers a lot of marginal improvements in a lot of places — like the browser — even if it makes a mess of some things. (gizmodo.com)
But with only marginal improvement over the fall semester, it is clear that the credit recovery program is key to making any significant increase before the spring semester concludes. (laschoolreport.com)
By 2000, pharmaceutical pipelines were filled with «me too» drugs that offered low risk and a quick route to approval, but only marginal improvement over drugs already available. (scientificamerican.com)