Variables: locally scoped variables ($variableName) use lower camelCase. Similar to fields, it's the $ which distinguishes them within calculation code.
$variableName
good
$variablename
bad
$variable_name
bad
Variables: global variables ($$VARIABLE.NAME) are capitalized and use domain specific prefixes relevant to their functional use within the solution. Each level of the domain is delineated using dot notation (a period).