One is a pronoun in the English language. It is an indefinite pronoun. For purposes of verb agreement it is a third-person singular pronoun, although it is sometimes used with first- or second-person reference. Occasionally it is called an impersonal pronoun. It is more or less equivalent to the Scotch "a body," the French pronoun on, the German/Scandinavian man, and the Spanish uno. It has the possessive form one's and the reflexive form oneself. The pronoun one has quite formal connotations, and is frequently avoided by the vulgar in favour of more colloquial alternatives such as the generic you.
One song can spark a moment,
One whisper can wake the dream.
One tree can start a forest,
One bird can herald spring.
One smile begins a friendship,
One moment can make us love.
One star can guide a ship at sea,
One word can frame the goal.
One vote can change a nation,
One sunbeam lights a room
One candle wipes out darkness,
One laugh will conquer gloom.
One step must start each journey.
One word must start each prayer.
One hope will raise one's spirits,
One touch can show one cares.
One voice can speak with wisdom,
One heart can know what's gone,
One life can make a difference,
So, ultimately, it's up to one!
(With apologies to Ashish Ram)