What is Filter Corner?

I was confused with the concept of the filter corner.
What is filter corner mentioned in the following picture?

Check out the wikipedia page on Butterworth filters, or a textbook on digital signal processing - one (of probably many) useful applied text for seismology is Gubbins, Time Series Analysis and Inverse Theory.

Is “corner” synonymous to “order” of the filter here? My confusion is that, being not a signal-processing expert, the sources I read, including the two you pointed out, do not use the word “corner” but instead they talk about the “order”. My understanding is that these are the same? For example, the figure here (from Gubbins) shows corners = 1, 2, 4, 6 and essentially higher “corner” number means sharper roll-off, and the filtered data should eventually stop changing at some value of “corner” .
Just want to confirm if my understanding is correct.

Exactly, “corners” is synonymous to “order” of the filter. Only for zerophase=True the final filter order is twice the value of corners.

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