Would this be defined as flirting?

Unless he takes action, no. In all honestly people are tempted every day by other people. You have to admit you've been attracted to other guys besides your husband. Thing is, even when people are flirting obviously, verbally and all, it doesn't mean they're going to kiss and what not. It doesn't mean he's unfaithful to you. It means that he's male and he's attracted to a younger women, what else is knew? Tell him you're uncomfortable if they start to spend more time together. Or you can bring it up earlier. Some people also lick their lips out of habit or if they're nervous/awkward. Unfortunately you noticing this may make you seem "insecure", so don't use this as an excuse to lash out at him. In a relationship as strong as MARRIAGE you BETTER feel comfortable talking to him about it though. So you could be like "So what do you think of the new girl? I noticed you were looking at her ;)"