Color Temperature Blue (alternatively, Correct to Blue). A range of color correction filters used to raise a source's color temperature. Available from all four major color filter manufacturers and also available in dichroic filters.
Full CTB will convert 3200K to 5700K
1/2 CTB will convert 3200K to 4300K
1/4 CTB will convert 3200K to 3600K
1/8 CTB will convert 3200K to 3300K
Some also offer 1/3, 2/3, and 3/4 varieties.
For more, see the links and attachments to the wiki entry color temperature. For the opposite, see CTO. Note that not all manufacturer's products are exactly equivalent.