「千」(sen) means "thousand" or "1,000". Similar to "百", its pronunciation changes in some combinations (e.g., 3,000 is 三千, sanzen). It's used for larger numbers, like money or quantities. Example: 「千円」(sen-en) means "one thousand yen".

「千(せん)」は「1,000」を意味(いみ)します。大(おお)きな数(かず)を表(あらわ)す時(とき)に使(つか)います。例(れい)：「千円(せんえん)」。