The phrase "regression line" refers to a line that represents the general trend or pattern of a set of data points. It helps to predict or estimate the values of the dependent variable based on the values of the independent variable.
To illustrate this interaction, the Figure 1 shows separate estimated regression lines for these 2 treatment groups for maltreatment involving the study child. (jamanetwork.com)
In particular, I'm interested because you talk about confidence intervals round regression lines. (wmbriggs.com)
A least - squares fit regression line for the simulations (solid line) and the observed seasonal cycle Δαs / ΔTs value based on ISCCP and ERA40 reanalysis (dashed vertical line) are also shown. (ipcc.ch)