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> I'm wondering if aphantasia is merely the inability to induce sensory hallucinations at will.

I have aphantasia and this is how I'd define it for myself. I have visual dreams and sometimes when I'm tired or on certain drugs I experience visualizations, but not being able to call these visualizations to mind deliberately I feel is what sets my experience apart from others.

I only have visual aphantasia though and can imagine sounds in my head quite well (with verification of this through being a musician and being able to play by ear), so the effect is definitely not universal and can be limited to certain senses.