请问regressor这个词应该怎么理解

大家好！请问regressor这个词应该怎么理解啊？我最近在很多地方都看到这个词，请大家不吝赐教。
在SPM8的manual里第62页讲到1st level analysis的Timing parameters设置时，提到“Also, with long TRs you may want to shift the regressors so that they are aligned to a particular slice. This is eected by changing the microtime resolution and onset.”

第63页的fig8.2也提到了这个词，图如下：

P63.Figure 8.2: Design matrix for fMRI data from two sessions. There are 24 experimental conditions for each session. The last two columns model the average activity in each session, giving a total of 50 regressors. There are 191 fMRI scans for each session. The overall design matrix therefore has 382 rows and 50 columns.

请问，regressor这个词该怎么理解？另外上面这幅图中的设计没有看明白，请指教。
谢谢！！