The Alay or Alai Mountains is a mountain range

Mountain range

A mountain range is a single, large mass consisting of a succession of mountains or narrowly spaced mountain ridges, with or without peaks, closely related in position, direction, formation, and age; a component part of a mountain system or of a mountain chain...

 that extends from the Tien Shan mountain range in Kyrgyzstan

Kyrgyzstan

Kyrgyzstan , officially the Kyrgyz Republic is one of the world's six independent Turkic states . Located in Central Asia, landlocked and mountainous, Kyrgyzstan is bordered by Kazakhstan to the north, Uzbekistan to the west, Tajikistan to the southwest and China to the east...

 west into Tajikistan

Tajikistan

Tajikistan , officially the Republic of Tajikistan , is a mountainous landlocked country in Central Asia. Afghanistan borders it to the south, Uzbekistan to the west, Kyrgyzstan to the north, and China to the east....

. It is part of the Pamir-Alay

Pamir-Alay

The Pamir-Alay is a mountain system in Tajikistan, Kyrgyzstan and Uzbekistan, part of the Pamir Mountains. It stretches between the valleys of the rivers Syr Darya to its north and Vakhsh to its south...

 mountain system. The range runs approximately east to west. Its highest summit is Pik Tandykul

Pik Tandykul

Pik Tandykul is a mountain in the Alay Mountains of Central Asia. Tandykul has an elevation of and is located on the international border between Kyrgyzstan and Tajikistan....

 , 5544 m. The southern slopes of the range drain into the Vakhsh River

Vakhsh River

The Vakhsh has been intensively developed for human use. Electricity, aluminum, and cotton are the mainstays of Tajikistan’s economy, and the Vakhsh is involved with all three of these sectors. Hydroelectricity provides 91% of the country’s electricity as of 2005, and 90% of that total comes from...

, a tributary of the Amu Darya

Amu Darya

The Amu Darya , also called Oxus and Amu River, is a major river in Central Asia. It is formed by the junction of the Vakhsh and Panj rivers...

. The streams that drain the northern slopes of the range are tributaries of the Syr Darya

Syr Darya

The Syr Darya , also transliterated Syrdarya or Sirdaryo, is a river in Central Asia, sometimes known as the Jaxartes or Yaxartes from its Ancient Greek name . The Greek name is derived from Old Persian, Yakhsha Arta , a reference to the color of the river's water...

, and empty into the Fergana Valley

Fergana Valley

The Fergana Valley or Farghana Valley is a region in Central Asia spreading across eastern Uzbekistan, Kyrgyzstan and Tajikistan. Divided across three subdivisions of the former Soviet Union, the valley is ethnically diverse, and in the early 21st century was the scene of ethnic conflict...

 to the north of the range.

Some sources seem to use the term for the whole southern curve of the Tian Shan

Tian Shan

The Tian Shan , also spelled Tien Shan, is a large mountain system located in Central Asia. The highest peak in the Tian Shan is Victory Peak , ....

corresponding to the southern border of Kyrgyzstan.