Fit

Fit[data,{f1,…,fn},{x,y,…}] finds a fit a1⁢f1+…+an⁢fn to a list of data for functions f1,…,fn of variables {x,y,…}. 

Fit[{m,v}] finds a fit vector a that minimizes ||m.a-v|| for a design matrix m.

Fit[…,"prop"] specifies what fit property prop should be returned.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.