prefix
Prefix used with direction words to mean 'exactly' or 'directly', e.g. 真上 (まうえ) 'directly above', 真東 (まひがし) 'due east'.
See also: 真上 (まうえ)
真上から太陽が照りつけている。
The sun is beating down from directly above.
この道を真っすぐ行くと、駅に出ます。
If you go straight along this road, you'll come out at the station.
prefix
Prefix that intensifies adjectives or nouns to mean 'completely' or 'truly', often with a geminate consonant, e.g. 真っ白 (まっしろ) 'pure white', 真っ暗 (まっくら) 'pitch dark'.
See also: 真っ白 (まっしろ)
外は真っ暗で何も見えない。
It's pitch dark outside and I can't see anything.
彼の言うことは真っ赤な嘘だ。
noun
Noun meaning 'truth' or 'reality', often used in set phrases like 真に受ける (まにうける) 'to take seriously' or 真に迫る (まにせまる) 'to be realistic'. Not commonly used as a standalone noun in modern Japanese.
See also: 真に受ける
彼の冗談を真に受けて怒ってしまった。
I took his joke seriously and got angry.
この映画の演技は真に迫っている。
The acting in this movie is very realistic.
What he says is a complete lie.
本当 is the common word for 'truth' or 'reality', while 真 as a noun is rare and mostly found in fixed expressions.
真実 is a more formal noun for 'truth', whereas 真 is a bound or archaic noun.
The prefix ま is native Japanese, likely related to the adjective 真 (まこと) 'truth' or the idea of 'true, real'. The exact derivation is uncertain.