Coptic is an alphabet which was and is used to write Egyptian. Around 150 BC, Egyptians were writing Egyptian using the Greek script, occasionally with some Demotic characters for sounds that weren’t in Greek. By around 300 AD, they had standardized on 24 characters from Greek and 6 from Demotic (with one more Demotic character in one of the Coptic spoken dialects). It also had one glyph for the syllable “ti” (made of a ligature of “t” and “i”).
Punctuation was not standardized, however. Some dialects of the script used an apostrophe-looking thing to separate words; some used umlaut-looking things to mark the beginning of some syllables; some used a circumflex-looking thing to mark syllables.
Coptic is also one of the few languages to have an upper and lower case.
Coptic was very important in linguistic history because it allowed scholars a bridge to the ancient Egyptian (which until the Coptic script had been written in logograms and consonants). It became closely tied with the Christian Church, and is still used today as a liturgical language by Egyptian Christians. This makes it one of the oldest continuously written languages still in use today after Chinese, Greek, Latin, Aramaic, and Hebrew.