How does the Enrollment_Term__c link to the Term record in Salesforce? So your XYZ object has a link to the Term__c but the Task's Enrollment_Term__c is not a Term__c Id... what is it?

In any case that question should give you the direction you need to go... try something like this (you'll have to fix it to work with your objects)

trigger createleadinterest on Task (after insert,after update) {
    List <XYZ__c> leadinterest = new List <XYZ__c>();
    Map<String, Term__c> termMap = new Map<String, Term__c>{};
    
    for (Task tk : Ttrigger.new) {
        if (tk.Type == 'Web Form' && tk.Enrollment_Term__c != null)
        {
            termMap.put(tk.Enrollment_Term__c);
        }
    }
    
    //Change this to the appropriate object name and filter criteria...
    for (Term__c item : [ select Id, Name from Term__c where Name IN :termMap.keySet()) ]
    {
        termMap.put(item.Name, item);
    }
    
    for (Task tk : Trigger.new) {
             
        if (tk.Type == 'Web Form') {
            
            XYZ__c ld = new XYZ__c (); //instantiate the object to put values for future record
            
            
            ld.Primary_Source__c = tk.Task_Source__c; // and so on so forth untill you map all the fields. 
            
            Term__c term = termMap.get(tk.Enrollment_Term__c);
            
            if (term != null)
            {
                ld.Term__c = term.Id; // Term__c is a look up field on XYZ__c 
            }
            
            leadinterest.add(ld);
            
        }
        
    }
    
    try {
        insert leadinterest; 
        } catch (system.Dmlexception e) {
        system.debug (e);
    }
    
}

 Again this will probably not compile for you but should give you a better idea of what you need to do.