バイト is the common short form of アルバイト. It means a part-time job, part-time work, or sometimes the part-time worker.
Common patterns:
• バイトする: to work part-time
• バイトがある: to have a part-time shift or job that day
• 新しいバイト（あたらしいバイト）: the new part-timer, or a new part-time job depending on context
バイト often sounds like work done by a student, younger person, or temporary worker.
パート also means part-time work, but it often suggests an adult worker, especially someone working shorter hours around home or family responsibilities.
In Japan, バイト is clearly different from 正社員（せいしゃいん）, a regular full-time employee. バイト work is usually seen as more temporary, with less status, fewer benefits, and less job security.