close relationship; deep connection; deep involvement
Used for personal bonds, business ties, or deep involvement in an activity. Often implies a relationship beyond the superficial.
I have a long-standing close relationship with him.
He is deeply involved in that incident.
密接 is a na-adjective/noun meaning 'close' or 'tight', often used for connections or links, but 深い関係 focuses more on the depth of a relationship rather than just closeness.
Compound of 深い (fukai, 'deep') and 関係 (kankei, 'relationship, connection'). The phrase is transparent and follows standard Japanese compounding rules.