Mathematical Bold Small Phi

It marks a Greek phi variable in bold mathematical style.

Some scientific notation uses bold or bold italic letters for vectors, but conventions differ by field. The equation or its notation key tells you whether 𝛗 means a vector here.

Where does Greek phi come from?

Greek added Φ for a p followed by a puff of breath.

The Greek alphabet adapted Phoenician writing, a script that mainly recorded consonants; Greek reused some letters for vowels. Its earliest surviving inscriptions date to around 760 BCE, and the exact origin of added Φ is disputed.

From Classical Greek to Modern Greek, phi’s sound shifted from p with extra breath to the f sound. By the early 20th century, plain φ was also used as a symbol for the golden ratio, a fixed proportion in geometry; 𝛗 has that meaning only when its equation assigns it.

Why does phi look different in fonts?

Fonts can show Greek phi with looped or straight forms.

Ordinary lowercase φ has glyph variation: some fonts use a looped shape, others the straight shape shown separately as ϕ. Unicode gives them distinct mathematical identities when notation needs that contrast: 𝛗 is based on φ, while 𝛟 is based on ϕ.