I am sending an email to users on regular basis using appian. I want to make a t

Try using an email template, and set your variable is an input parameter to the template. More info here:
forum.appian.com/.../Send_Email_Smart_Service

yes thats exactly what I am using. How do I make the variable I am passing as bold?

In the Send Email node, when configuring the body, you should be able to use "Find in template document" and "Replace with" to do find/replace. If you are replacing the word "var" in your template with the variable, make sure the template has <b>var</b>

I see thanks.

You have two options and either should work depending on your requirements; the first is to make the field itself bold on the template side (using standard html tags); the other would be to pass the HTML "bold" tags into the template variable in the Send Email node. Such as with a pv called "myVariable", in the Send Mail node for that template field you'd put the following:
="<b>" & pv!myVariable & "</b>"

Currently I am passing a whole chunk of data in one variable to a variable in an email template. I want some text in that variable to be bold. I don't want to create so many variables just for this purpose. Is there any way around this?

As long as you have a way to insert the HTML <b> tags around the text you want bolded, they can go anywhere in the text being passed into the Email (my use of a special variable was just for the purposes of the most simple example).