Dual-status alien with regards to taxes.
Dual-status aliens with regards to taxes are aliens who are considered non-residents part of a year, and residents the other part. In most countries this situation requires special reporting and special rules on how to calculate taxes and what income is taxed when.
In the US, dual-status aliens are aliens who arrived to the US for a prolonged/permanent periods of time, during their first year in the US (the year of the arrival). In some cases being dual-status is a choice, in others there's no way around it. This is a common situation for new immigrants, H1b visa holders and L1 visa holders.