A quantitative metric that measures how well a particular mask or respirator fits to a specific individual's face, defined as the ratio of the ambient concentration of aerosol particles to the concentration within the facepiece during use.

(mathematics) A line on a scatter plot that best defines or expresses the trend shown in the plotted points. It is chosen so that the sum of the squares of the distances from the points to the line is a minimum. It also illustrates the correlation (positive, negative, or zero correlation) between the plotted variables.

(simile) To be a perfect fit, to be exactly the right size.

(chiefly Britain) Very fit; in good shape.

(idiomatic) If a description fits, it is probably the case; if an insult accurately describes you, you must bear it; if you take offence at an insult that was only implicitly directed at you, you admit the insult is true enough to be recognizable.

(simile, Australia) In good physical health.

(Australia) a health indicator age in comparison to a person's true age, based on a survey of diet, exercise, and other factors.

(idiomatic) At a very high volume; extremely loudly.

plural of line of best fit

(Britain) Alternative form of if the shoe fits, wear it