Xilinxuses a different term inXPS_IICdata sheet:
Throttle Description
The Philips I2C-bus Specification permits devices to throttle (suspend)
data transmission on the bus by holding the SCL line low for an indefinite
period of time.
(DS606 December 2, 2009, page 24)
From the Philips spec:
3.9 Clock stretching
Clock stretching pauses a transaction by holding the SCL line LOW. The
transaction cannot continue until the line is released HIGH again.
(UM10204, I2C-bus specification and user manual, Rev. 03, 19 June 2007,
http://www.nxp.com/acrobat/usermanuals/UM10204_3.pdf)