Triquetra interlaced with a circle (ribbons or pseudo-Celtic knotwork).
This is a traditional Christian Trinitarian symbol, but similar signs are also sometimes used by new agers or neo-pagans (though more often in non-interlaced form, as in Zoso.svg, Triquetra on book cover.jpg, or Triquetra.jpg).  In this interlaced form, can be known as the "Trinity knot".
(Christian Symbolism:
The Triquetra represents the Holy Trinity: the Father, Son, and Holy Spirit. The unbroken circle represents eternity. The interwoven nature of the symbol denotes the indivisibility and equality of the Holy Trinity. It symbolizes that the Holy Trinity is three beings of power, honor, and glory but is indivisibly one God.)

For other versions of the triquetra, see also Triquetra-Vesica.png , Triquetra-Vesica-solid.png , Triquetra-tightly-knotted.png , and Triquetra-Double.svg , and Triquetra-Interlaced-Triangle-Circle.svg .
Vector version of Triquetra-circle-interlaced.png, converted from the following PostScript code:

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/l{stroke}!/S{3 sqrt mul}!/k{0 0 0 s 5.6 setlinewidth}!
/C{50 50 3 sqrt div 19.4}!/K{1 1 1 s 2.8 setlinewidth}!
/C2{25 3 sqrt div}!/C3{25 8.5 cos div}!
/C4{C3 8.5 sin mul}!/C5{50 C2 C4 sub C3}!
/C6{62.5 C4 S 2 div add C2 12.5 S add C4 2 div add C3}!
/C7{37.5 C4 S 2 div sub C2 12.5 S add C4 2 div add C3}!
k C 0 360 arc closepath l K C 0 360 arc closepath l
k C5 4.0925 175.9075 arc l C7 -115.9075 55.9075 arc l
C6 124.0925 -64.0925 arc l K C5 6.149 173.851 arc l
C7 -113.851 53.851 arc l C6 126.149 -66.149 arc l
k C5 106 117 arc l C7 -14 -3 arc l C6 226 237 arc l
C 48 80 arc l C 168 200 arc l C 288 320 arc l
K C5 105 118 arc l C7 -15 -2 arc l C6 225 238 arc l
C 46 82 arc l C 166 202 arc l C 286 322 arc l showpage
%EOF
AnonMoos (initial SVG conversion of PostScript source by AnonMoos was done by Indolences)