The term "400th" refers to the ordinal number that indicates the position of something in a sequence, specifically the four hundredth item. It is often used to mark significant milestones, such as anniversaries or achievements. For example, a celebration might occur for the 400th anniversary of a historical event or the 400th publication of a book. In mathematics, "400th" can also relate to fractions or percentages. For instance, if a group has 1,600 members, the 400th member would represent 25% of the total. This concept is applicable in various fields, including education, sports, and science.