add central tendency to glossary (see #2316, #2314)

lessons/Data-Science/measures-of-center/langs/en-us/index.adoc

@@ -59,13 +59,14 @@ When working with dot plots, we often wondered, "What is a typical value in this
 ... and we often discovered that there were a variety of correct responses!
 
 When thinking about typicality, there are a few approaches:
+
 - identify the value that is most common
 - identify the midpoint of the sample
 - identify a "balance point" among the data points
 
 It turns out that these different ways of thinking about typicality are all different ways of "measuring the center".  "What value is typical?" is asking "is there a value that all the others tend to cluster around?"
 
-Statisticians have a more formal, mathematically-defined term for this: central tendency. Central tendency is a "summary" measure, that attempts to describe a whole set of data with a single value that represents the middle or "center" of its distribution. In other words, _a value that all the others tend to cluster around_.
+Statisticians have a more formal, mathematically-defined term for this: @vocab{central tendency}. Central tendency is a "summary" measure, that attempts to describe a whole set of data with a single value that represents the middle or "center" of its distribution. In other words, _a value that all the others tend to cluster around_.
 
 There are several different measures of central tendency. Data Scientists know what each measure is, and when to use it. And in the next few lessons, you'll learn that too.
 

lib/glossary-terms.json

@@ -220,6 +220,15 @@
     }
   },
   {
+    "en-us": {
+      "keywords": [[ "central tendency" ]],
+      "description": "a summary measure, that attempts to describe a whole set of data with a single value that represents the middle or center of its distribution. In other words, a value that all the others tend to cluster around."
+  },
+    "es-mx": {
+      "keywords": [[ "" ]],
+      "description": ""
+    }
+  },{
     "en-us": {
       "keywords": [[ "circle of evaluation", "circles of evaluation" ]],
       "description": "a diagram of the structure of a mathematical expression (Bootstrap-specific)"