What 𝐞 means
It marks an e with a separate role in that formula.
A 1999 proposal for Unicode's mathematical alphabets expanded an older, smaller set of letterlike symbols: characters shaped like letters that already served as mathematical signs. The expansion let plain text preserve distinctions between formula symbols.
The conventions around 𝐞 in a particular formula tell the reader which role the writer intends.
Use text formatting for ordinary emphasis
For visual emphasis in an ordinary sentence, apply bold styling to plain e. Mathematical bold letters are intended for mathematical or technical notation, where weight can distinguish symbols.
In Unicode 18.0, NFKC normalization can turn 𝐞 into plain e. NFKC is a text-normalization form that removes compatibility distinctions; that can erase a difference a formula needs, so avoid it for mathematical text.
Don't confuse it with
1Questions people ask
1Why does 𝐞 show as a box on some devices?
Because that device's fonts have no glyph for it. The character is still correct and still copies correctly — the box is a missing glyph, not a broken character.
How to type 𝐞
𝐞\1D41ECodes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.