You call an incomplete cheque a "blank cheque" most of the time. Another less frequently used term is "inchoate cheque".
What's a blank cheque?
A blank cheque is a cheque that has not had a numerical value written on it, but is already signed by the account holder. This usually occurs when there has been an agreement between to parties that a figure will be added at a later date.
The issues with blank cheques
Although blank cheques may make business transactions easier, there are some potential issues when dealing with blank cheques. The fact that they're already signed means that the person with the cheque can write any figure they like on it, and cash it in for that value.
There needs to be a high level of trust between individuals before a blank cheque is handed out, and it's certainly not something I'd ever consider doing myself. I'd be too worried about losing a lot of money to someone that's willing to abuse the blank cheque agreement. Okay, maybe I'd let my family have a blank cheque...