adverb
Used to reveal the true situation, often contrasting with appearances or assumptions. Common in written and formal spoken Japanese.
彼は若く見えるが、その実、五十歳を超えている。
He looks young, but in fact he is over fifty.
その実、彼女は誰よりも努力していた。
In reality, she was working harder than anyone.
実際 is more common in everyday speech and can also mean 'practical' or 'actual'. その実 is more literary and emphasizes a contrast with surface appearances.
実は is a conversational opener meaning 'actually' or 'as a matter of fact'. その実 is used within a statement to reveal the truth, not as a sentence starter.
Compound of その (that) and 実 (truth, reality). The reading じつ for 実 is the standard on'yomi.