Heb 4:12 (KJV1611 Edition):
For the word of God is quicke and powerfull,
and sharper then any two edged sword,
pearcing euen to the diuiding asunder of soule and spirit,
and of the ioynts and marrowe,
and is a discerner of the thoughts and intents of the heart.
There are two Greek words translated 'the word':
Logos - the living Word of God, the person: Messiah Jesus
Rhema - the written Word of God, the books: the Holy Bible
The term here is 'Logos', so it is speaking of Messiah Jesus.
Here is the entry from Strong's:
G3056
λόγος
logos
log'-os
From G3004; something said (including the thought); by implication
a topic (subject of discourse), also reasoning (the mental faculty)
or motive; by extension a computation; specifically
(with the article in John) the Divine Expression (that is, Christ):
- account, cause, communication, X concerning, doctrine, fame,
X have to do, intent, matter, mouth, preaching, question,
reason, + reckon, remove, say (-ing), shew, X speaker,
speech, talk, thing, + none of these things move me,
tidings, treatise, utterance, word, work.