From Teflpedia

A ligature (/lɪgəʧə/) occurs where two or more graphemes (especially letters) are joined as a single glyph.

Examples in English include &, and sometimes Æ, Œ. This is rare in contemporary English, less so in some forms of historical English.

The phonemic symbols /ʧ/ and /ʤ/ are also ligatures of /t/+/ʃ/ and /d/+/ʒ/ respectively.