What do you call the phenomenon where a rectangle □ is shown because a font lacks a glyph?

Solution 1:

The Unicode standard calls it a replacement glyph.

Solution 2:

Google calls them 'tofu', too. They have a Google Font called Noto which is named (see the link) for 'no to tofu'.

When text is rendered by a computer, sometimes there will be characters in the text that can not be displayed, because no font that supports them is available to the computer. When this occurs, small boxes are shown to represent the characters. We call those small boxes “tofu,” and we want to remove tofu from the Web.

Solution 3:

Glyph Replacement is too general a description (sorry, Unicode). That could mean that a font replaces 'HEAVY BLACK HEART'❤ with 'BLACK HEART SUIT' ♥ or even ampersand with plus. The phenomenon is not replacement, it is Glyph Failure. In practice, the □ codepoint is used as a Failure Glyph.