在這句中，stop是甚麼詞性？

詞性

In "stop payment", 'stop' is a transitive verb and 'payment' is a noun and the object.
In "stop payment orders", "stop payment' acts as an adjective describing "orders".
On the other hand, "stop payment orders" is the object for the gerund 'effecting' which is the present participle of the transitive verb 'effect'.

2012-10-29 11:59:28 補充：
I don't agree with the above because 'stop' as an adjective to describe 'payment does not make sense. We can say cash payment, cheque payment. However 'stop' means an action to stop the payment, not describing it as there is actually no payment.

I agree with Ahshing. In this particular case "stop" is the first "noun" of the compound noun "stop payment", and like most of the compound noun relationships, serves a function similar to an "adjective", to describe the second noun "payment".

2012-10-31 13:20:09 補充：
I agree "stop" is indeed a verb here, but is used as a component of the compound noun "Stop Payment". Compound nouns can consist of 2 nouns, or other combinations, such as verb / noun (swimming pool), adjective / noun (full moon) etc.

stop payment = stop is telling us more about what kind of payment.

stop and payment are both nouns
Stop(adj) payment allowed
Stop payment means stoppage (n) to the payment
=the act of being stopped
=a cessation of working eg. in a cheque;
=a deduction from pay(---) to the cheque
=a service charge.

STOP PAYMENT is a compound noun with both STOP and PAYMENT both being a noun.

STOP PAYMENT = An order to a bank not to honor the payment of a cheque after it has been delivered but before it has been cashed.