domasla is right as to scissors always in plural form. As a matter of fact, each of the two blades of the tool is known as 'scissor' by theory... The same as the "cutting shears", the last letter 's' also denotes the plurality of the tool. Thus, "A pair of scissors IS" , or "THESE scissors ARE" are most likely to be used. :)

It is always plural, because it's made of two parts. A (/one) pair of scissors. Two pairs of scissors. Three pairs of scissors. 'A scissor' is or 'a scissors' not correct.

Scissor is quantitative, it is countable. But a single scissor is formed in pair. So, one scissor is a pair of scissor, more than one is: two pair of scissors, three pair of scisors., and so on...

Here's a dictionary definition; scis·sor (szr) tr.v. scis·sored, scis·sor·ing, scis·sors To cut or clip with scissors or shears. n. 1. scissors (used with a sing. or pl. verb) A cutting implement consisting of two blades joined by a swivel pin that allows the cutting edges to be opened and closed. 2. scissors (used with a sing. verb) Sports a. Any of various gymnastic exercises or jumps in which the movement of the legs suggests the opening and closing of scissors. b. A scissors hold.