If you say that something is true or seems to be true at first glance, you mean that it seems to be true when you first see it or think about it, but that your first impression may be wrong.

What is at first glance mean?

DEFINITIONS1. at the moment when you first see something or find out about something, before you know more details. At first glance, the theory seems to make a lot of sense.

Is it at first glance or at a first glance?

The Google Ngram Viewer shows at first glance is much more frequently used than at a first glance. Furthermore, as in this link(OALD), at first glance is entered as a stand-alone idiom.