Forethought is careful thinking you do before you act — packing a coat because you used forethought about the rain, saving a little money with some forethought. It is the opposite of rushing in. The word is a little formal and a little old-fashioned, but people still use it, often in the phrase with / without forethought. Though actually, in everyday talk many people say planning ahead or thinking ahead instead. Afterthought is the sibling word: something you only think of later. There is also a rare old verb, forethink, but you can live without it.