Sometimes it's 51%, sometimes it's 66%, sometimes it's 100%. But what do you call that number?
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Presumably OP would like something along the lines of threshold
the magnitude or intensity that must be exceeded for a certain reaction, phenomenon, result, or condition to occur or be manifested.
I think this is a perfect fit.
It's called a supermajority:
su·per·ma·jor·i·ty (so͞o′pər-mə-jôr′ĭ-tē, -jŏr′-)
n. pl. su·per·ma·jor·i·ties
A specified majority of votes, such as 60 percent, required to approve a motion or pass legislation.
[supermajority. (n.d.) American Heritage® Dictionary of the English Language, Fifth Edition. (2011). Retrieved September 20 2015 from http://www.thefreedictionary.com/supermajority]
Note: Considering only the titular question and disregarding the examples in the body of your question, the answer would be the self-evident "decisive percentage". To me, that answer is unfairly dismissive of the examples you gave in the body of the question.
Edit: Response to Comments
Because the framing of your question indicated they might be appropriate, my original answer of 'supermajority' is based on these assumptions:
I assume you are asking about decisions reached on the basis of some type of majority because all of your examples are greater than 50%.
I assume you are asking about a subtype of 'majority', because all of your percentage examples are greater than 50%.
I assume you are not asking about a simple majority (50% plus 1 vote), because 51% is only a simple majority in the case where only 100 votes are in play.
A. From the comments of others, and particularly, deadrat's, I gather that assumption 3 may be unjustified. Deadrat's opposing assumption is that by "51%" you intended "50% plus 1 vote". While deadrat may well be right (you can read that as 'almost certainly is, now that I think about it' if you want), this is a point for you to decide and clarify if you choose.
B. From your comments, I understand that, whatever your original question might have been, you are also interested in what the term for a decisive percentage would be in the case where that percentage is less than 50%.
Case 1: Regarding A, and if my assumption 1 is correct (but not 2 and 3), the answer to your question would be 'majority': the percentage needed to reach a decision is only a majority percentage.
Case 2: Regarding A, and if my assumptions 1 and 2 (but not 3) are correct, the answer depends on the percentage: if the deciding percentage is 51% (=50% plus 1 vote), the answer is a 'simple majority' (note that a 'simple majority' is also often called a 'bare majority'); if the deciding percentage is 60%, the answer is 'supermajority'; if the deciding percentage is 100%, 'supermajority' will also serve, but 'absolute majority' is also often used in that case.
Case 3: Regarding A, and if my assumptions 1-3 are correct, the answer to your question is 'supermajority'.
From that breakdown regarding A, you can see that my original answer was more than half right: it was right in Case 3 and in 2/3 of Case 2. So my original answer, because it was more than half right, would itself reach a decisive, majority percentage of 'rightness'.
Regarding B: In the case where the deciding percentage is less than 50%, all of my stipulations (and answers) regarding A may also be true. The difference is in the type of decision being made and the system underlying the decision; these two considerations together determine the rules for what constitutes a 'majority', and in which circumstances.
The type of decision to be made might be unary, binary, ternary, ..., n-ary in terms of quantity (number of options or candidates available). Note that other types of decision are possible; for example, decision types might be considered in terms of value (yes, no, undecided, slightly favoring, etc.) rather than quantity; or decision types might be combinations of quantity and value.
The underlying systems may be, for example, and considering only two of the possible systems, plurality or majority systems.
For a plurality system, if the type of decision is quantitatively unary (there's only one candidate or option), any vote cast constitutes a majority and the single candidate or option gets the decision. If the type of decision is binary, the candidate or option with the majority of votes gets the decision. If the type of decision is ternary or greater, the candidate or option that gets more votes than any other single candidate or option gets the decision. That winning candidate or option, in a plurality system, need not get more votes than all of the opposing candidates or options combined.
For a majority system, if the type of decision is quantitatively unary, again any vote cast is a majority. If the type of decision is quantitatively binary, again, the decision goes to the candidate or option with the majority of votes. If type of decision is ternary, however, the decision goes to the candidate or option with more votes than all of the opposing candidates or options combined.
As you can see, in plurality systems (such as those of the US, Canadian and British national legislative elections), unless there is a tie, one candidate or option has a majority even when there are more than two candidates or options. That resolution by a majority vote is less common in majority systems. Such systems typically incorporate a mechanism or mechanisms for resolving votes (usually involving more than two candidates or options) that do not result in a decision because no single candidate or option gets more votes than all the opposing candidates or options combined.
For a much more lucid and complete description of plurality, majority and other systems see, for example, the discussion of "Systems of Vote Counting" at Encyclopædia Britannica Online (s. v. "election", accessed September 21, 2015, http://www.britannica.com/topic/election-political-science).