Google definition:
Tension in a literary context is
the sense that something ominous is right around the corner. Building a large amount of tension as a writer keeps your readers engaged up until the end of the story. ... Good use of tension makes a story worth reading and keeps readers guessing.
Also:
anticipation of conflict. The reader and the characters know a problem is on the horizon.
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changes that impact the novel’s characters. Changes that become more dire as the story progresses. No sooner is one obstacle conquered than another one crops up.
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a series of unexpected events or revelations. Often the events surprise the reader and the protagonist. Sometimes the reader knows about the event but the character doesn’t. This adds to reader tension.
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torment. The protagonist must suffer both internal and external agony. The obstacles your character has to face must continually increase in difficulty until he’s pushed to his absolute limits.
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secrets. Character must have needs, painful past experiences and skeletons in his closet that precipitate his reaction to events.
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dialogue. Not just idle conversation, but as Morrell puts it, “…conversation’s greatest hits.” Dialogue that is a power struggle where there is a winner and a loser.
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subtext. Unspoken thoughts, fears, anger and motives of characters—what they really think, feel and believe.
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ebbs and flows. There needs to be variations in the levels of tension throughout the story in order for readers to relax and prepare for the next jolt of stress they’ll experience.
Oda's done this. Otherwise you wouldn't still be reading the story right now.