While it might seem straightforward and obvious, I think there are multiple deeper meanings to be found with the title Allegiant.
I think it’s a trend you see throughout the series: “divergent” means “tending to be different or develop in different directions,” which describes how Tris is different than all of her peers by not exhibiting the traits just one of the factions; “insurgent” means “rising in active revolt,” which describes how Tris is fighting against Jeanine’s army.
And then you have the meaning of “allegiant”: “Loyalty or the obligation of loyalty, as to a nation, sovereign, or cause.”
What is interesting is how all of those words are technically adjectives, but are exclusively applied as nouns in the Divergent series, getting at the symbolic nature of what it means to be “divergent.”
Being different isn’t just something that describes you—when you have trouble fitting in, whether in school or at work or even in your own family, “divergent” very much becomes your identity. So Tris starts out as a Divergent, aka someone who doesn’t fit in. She sees all these different factions she’s “supposed to fit into” (just like there are jocks, cheerleaders, nerds, and burnout factions at every high school), but is simply her own person. And then, as I wrote in my piece on Insurgent, the next step is fighting those expectations, staying true to yourself and refusing to join any one faction because that’s what expected of everyone. In that sense, “insurgent,” just like “divergent,” becomes a noun: Insurgent becomes Tris’ identity. She embodies the idea of revolting.
Which brings us to Allegiant, which, following the trend of the other two films, becomes a noun: Tris embodies loyalty. But in Allegiant, the definition is a bit more open-ended, and her loyalty is to a “nation, sovereign, or cause.” We’ve had Divergent Tris, who is discovering how she is different than everyone else; then we have Insurgent Tris, who is fighting against societal norms; and then with Allegiant Tris, we see someone who is finally choosing a side to fight for.
Let’s think of it in terms of a high school student for one second. You get to school, realize you don’t fit in with any group, realize you’re Divergent; suddenly you’re getting pressured by groups, one of the popular groups starts to pick on you, but then you realize you’re a beautiful person and don’t need to belong to any one particular group—you’re Insurgent; now that you’ve transcended norms, you have power, people look up to you because they’re tired of belonging to groups, and even the other popular cliques start buddying up to you, and now you must choose if you will stay loyal to yourself or join another powerful group, testing what it means to be an Allegiant.
For Tris, she has to decide between trusting in the sovereign (David) or staying true to her plight. David might represent something bigger and better, but also might be playing her, abusing her status and influence. The journey of whom she remains “allegiant” to (between David and herself) gets at the core of the meaning of the title: Allegiant.