Ad hoc is latin for “for this purpose”. You might call it an “on the fly” query, or a “just so” query. It’s the kind of SQL query you just loosely type out where you need it
var newSqlQuery = "SELECT * FROM table WHERE id = " + myId;
…which is an entirely different query each time that line of code is executed, depending on the value of myId. The opposite of an ad hoc query is a predefined query such as a Stored Procedure, where you have created a single query for the entire generalized purpose of selecting from that table (say), and pass the ID as a variable.