The correct spelling for "ulterior" is U-L-T-E-R-I-O-R.
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An ulterior motive is a motive that they have beyond the main
motive. They often do not show that they have this motive and it is
often not good. The ulterior motive is almost always has it for
their own good only.
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Example sentence - It soon became clear he had ulterior motives
in regards to her future.