To truncate is to shorten by cutting off. In computer terms, when information is truncated, it is ended abruptly at a certain spot. For example, if a program truncates a field containing the value of pi (3.14159265...) at four decimal places, the field would show 3.1415 as an answer. This is different from rounding, which would result in the answer of 3.1416.
For another example, consider a text entry field in an electronic form. If a program limits the size of the field to 255 characters, it may allow you to continue typing past the maxiumum number of characters. However, when the information is submitted or saved, the program truncates the data to the first 255 characters and any additional characters are disregarded.
Several operating systems or programming languages use truncate as a command or function for limiting the size of a field, data stream, or file. Linux includes a truncate command for rounding integers.