Neutral tone 輕聲 (轻声)
qīngshēng · from Chinese
What is the neutral tone in Mandarin?
The neutral tone is a short, light, unstressed syllable in Mandarin whose pitch depends on the tone before it rather than having its own contour.
Often called the fifth tone, the neutral tone appears on grammatical particles, suffixes such as 子 zi, and the second syllable of many common words. It can distinguish meaning: 東西 dōngxī means east and west, while 東西 dōngxi means thing. Mainland Mandarin uses it far more than Taiwanese Mandarin, where many of these syllables keep a full tone.
Examples
媽媽 māma (mum); 東西 dōngxi (thing) vs dōngxī (east and west); 桌子 zhuōzi (table); 嗎 ma (question particle)
Kungfucius says: The tone that is so neutral it changes depending on whether you learnt Mandarin in Beijing or Taipei.