Japanese meter symbol (square mētoru)

It's "metre" in katakana, fitted into a single character cell. Japanese writes borrowed words such as metre in katakana, one of its two syllable scripts: メ (me), ー (a long-vowel mark), ト (to), ル (ru). Stacking them two over two lets the whole word occupy one square cell of the Japanese text grid, which is handy in narrow table columns, forms and vertical text.

The form came into computing through Japanese PC character sets of the 1980s: it is one of NEC's special characters, later carried into the Windows Japanese character set, and Unicode took it over in 1993 so old text converts without loss. It is a compatibility character: standard text normalization turns it back into the four ordinary katakana メートル, so search engines often treat the two as the same word.

Japanese style guides long classed it as a "machine-dependent character" (機種依存文字) that might not display on other computers. Most modern systems show it, though fonts draw it differently and an old font may show an empty box. For text that must be searched or read by non-Japanese readers, write メートル or simply m.